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velocity map imaging apparatus for studies on the photochemistry of

1. 68 Table 3 Translational and rotational temperatures and energies of CH v 0 products xiii Chapter 1 Introduction The investigation of the physical and chemical properties of vapor deposited ice has received much attention in the recent years primarily due to important astrochemical and atmospheric implications Water ice in the cold dense regions of interstellar clouds is a medium for photochemical reactions when exposed to ultraviolet UV radiation On earth polar regions act as sinks for atmospheric pollutants and UV photolysis of these compounds produces photoproducts that impact the environment For example heterogeneous reactions between water ice and molecules such as HCl CIONO and N2Os in the stratosphere play a central role in the occurrence of the Antarctic ozone hole Furthermore simple molecules such as CHOH NH3 CO H2 found on interstellar ices are considered to be the building blocks of the solar system as their interaction with UV radiation gives rise to complex molecules in molecular 9 10 clouds Therefore studies of ice photochemistry in the interstellar medium in the polar regions and in the laboratory are of great importance With the novel Velocity Map Imaging apparatus under development at Queen s University photochemical reactions on amorphous or polycrystalline ice and their contaminants can be studied in a controlled lab en
2. 57 Figure 14 The trajectories originate at y 0 mm with one ion ejected with a 90 elevation angle and other at 0 elevation angle 58 Figure 15 Schematic for the projection of ionized photoproducts originating at point O on to the detector 60 Figure 16 Typical image simulated by the Visual Basic program top using broadened velocity distributions the Inverted image from the Onion Peeling Program middle and the Inverted image from the Abel Transform bottom 67 Figure 17 Implementation of a for loop in LabVIEW Block Diagram and Front Panel 74 Figure 18 Communicating with an instrument 75 Figure 19 Schematic for the data acquisition system using V s 78 Figure 20 Communication with the device 79 Figure 21 Front panel of power supply 80 Figure 22 DG 535 front panel 83 Figure 23 DAQ in the PCI 6602E VI
3. 86 Figure 24 Front Panel of PCI 6602 VI 88 Figure 25 Stepper motor controller and data acquisition device 90 Figure 26 Block diagram to change direction of rotation in the stepper motor 91 Figure 27 Front Panel of the Stepper Motor sub VI 92 Figure 28 Front panel of the Stepper Motor Scope REMPI VI 93 Figure 29 Imaging VI front panel 1 for user inputs 97 Figure 30 Imaging VI front panel 2 to display images 97 Figure 31 Imaging VI block diagram 98 Figure 32 Evolution of features in CH3OH irradiation MF methyl formate 103 Figure 33 Schematic illustration of the experiment 106 Figure 34 Detection of the photofragment from the surface 109 Figure 35 a 2 1 REMPI excitation spectrum of CH radicals from the 157 nm photolysis of a fresh ASM sample at 90 K recorded at t 6 0 us b Simulated REMPI excitation spectrum of CH radicals 111 xi Figure 36 TOF spectrum of CH radicals from
4. Amorphous Solid Water ASW is a major constituent in interstellar clouds comets satellites of outer planets and icy grain mantles Thus it has important astronomical implications Also water ice in the cubic or amorphous phase has been reported as a major component of the surface of many satellites and planetary rings On earth heterogeneous reactions occurring on the surfaces of polar stratospheric cloud particles are recognized to play a central role in the photochemical mechanism responsible for the occurrence of the Antarctic ozone hole The crystallization process of ASW is important in physical phenomena associated with ices such as sublimation the outgassing of volatile molecules and changes in the thermal conductivity These properties are controlled to a large extent by changes in the hydrogen bonded network of the water during heating By characterizing the structural changes that occur within water ice it may be possible to understand the chemistry of cometary and interstellar ice and stratospheric ice particles Hence the study of water ice at low temperatures and the ability of water ice to trap gases have received much attention Photochemistry of water ice plays an important role in interstellar grain chemistry For this reason primary and secondary photodissociation reactions taking place on water have received extensive attention because of their importance in atmospheric chemistry and astrophysics Thi
5. Rep FIG 18 Communicating with an instrument 75 The instrument I O tools in LabVIEW create the components required to communicate with an external device attached to a serial or a GPIB port and provides the ability to switch between interfaces This chapter discusses the programming of the instruments used in the experiment by using LabVIEW 8 6 The experimental details of velocity map imaging have been discussed in detail in chapter 3 An overview of the experimental section will be presented in the following section in light of the programming process The main elements of the programming environment are a pulsed laser system a Molecular Beam Valve MBV and a charged particle detection system The pulsed laser system is comprised of a photolysis laser Lambda Physik LPX200 Excimer Laser and a probe laser Scanmate Lambda Physik Dye Laser pumped by a Nd YAG Laser Continuum The timing of the photolysis and probe lasers is achieved through a delay generator DG 535 Stanford Research Systems and a PCI6602 timing board via the DG535 VI and the PCI6602 VI The latter is used to control the MBV as well The ions generated in the vacuum chamber are accelerated through an ion optics assembly and pass through a time of flight TOF region before hitting the MCP The high voltage applied to the ion optics assembly and the MCP Chevron configuration Burle Electro Optics Inc is controlled by the PS350 High Voltage Power Supply VI
6. HR H H gt H 3 7 More over the OH photoproducts formed by the 157 nm photodissociation of ASW at 90 K can recombine to produce H202 or may produce HO as given in reactions 3 9 and 3 10 OH OH H0 3 9 OH H 0 H20 HO 3 10 A majority of the studies on the photolysis of ASW and its constituents have been directed towards investigating the products and the yields of reactions The photolysis in the condensed phase is more complicated than in the gas phase as a result of the secondary photo processes that take place in the ice matrix Therefore it is important 125 investigate the possible reaction channels in the energetic process An understanding of the kinetic energy and internal energy distribution of the photoproducts would be imperative to provide a complete understanding of the reaction mechanisms 126 References for Chapter 5 T Hama M Yokohama A Yabushita M Kawasaki P Wickramasinghe W Guo H P Loock M N R Ashfold and C M Western J Chem Phys 131 224512 2009 P A Gerakines W A Schutte and P Ehrenfreund Astron Astrophys 312 289 1996 G A Baratta G Let and M E Palumbo Astron Astrophys 384 343 2002 C J Bennett S H Chen BJ Sun A H H Chang and R I Kaiser Astron Astrophys 660 1588 2007 M J Mumma M A DiSanti K Magee Sauer B P Bonev G L Villanueva H Kawakita N D Russo E L Gibb G A Blake J
7. J Bass M Brouard A P Clark B Martinez Haya and C Vallence Phys Chem Chem Phys 5 856 2003 20S Manzhos and H P Loock Comp Phys Commun 154 76 2003 R H Brown and D P Cruikshank Annu Rev Earth Planet Sci 25 243 1997 2J E Schaff and J T Roberts J Phys Chem 100 14151 1996 3A Susa and S Koda Meas Sci Technol 15 1230 2004 aS Malyk G Kumi H Reisler and C Witting J Phys Chem 111 13365 2007 235D L Cocke J A G Gomes J L Gossage K Li C J Lin S Tandel App Spec 58 528 2004 D A Dahl J E Delmore and A D Appelhans Rev Sci Instrum 61 607 1990 ANG Romanescu S Manzhos D Boldovsky J Clarke H P Loock J Chem Phys 120 767 2004 28 Winterhalter D Maier J Honerkamp V Schyja and H Helm H J Chem Phys 110 11187 1999 PNJ Dribinski A Ossadtchi V A Mandelshtam and H Reisler Rev Sci Instrum 73 2634 2002 2O Ghafur W Siu P Johnson M F Kling M Drescher and M J J Vrakking Rev Sci Instrum 80 033110 2009 71 Chapter 4 Programming of Instrumental Components in the Experiment 4 0 Programming with LabVIEW Laboratory Virtual Instrumentation Engineering Workbench LabVIEW is a powerful tool developed by National Instruments NI that enables computerized control of instruments The building blocks used in LabVIEW programming are known as Virtual Instruments VIs The programming
8. M Dehmer and J L Dehmer Phys Rev A 43 4702 1991 DY Matsumi N Shafer K Tonokura M Kawasaki Y L Huang and R J Gordon J Chem Phys 95 7311 1991 2A Yabushita T Hama M Yokoyama M Kawasaki S Andersson R N Dixon M N R Ashfold and N Watanabe Astrophys J 699 L80 2009 IT Hama A Yabushita M Yokoyama M Kawasaki and N Watanabe J Chem Phys 131 114510 2009 2T Hama A Yabushita M Yokoyama M Kawasaki and N Watanabe J Chem Phys 131 114511 2009 BF M Zimmermann and W Ho J Chem Phys 100 7700 1994 F M Zimmermann and W Ho Surf Sci Rep 22 127 1995 Yabushita Y Inoue T Senga M Kawasaki and S Sato J Phys Chem A 108 438 2004 260p Quintana R F Delmdahl D H Parker B Martinez Haya F J Aoiz L Banares and E Verdasco Chem Phys Lett 325 146 2000 ly _P Kuo H C Lu Y J Wu B M Cheng and J F Ogilvie Chem Phys Lett 447 168 2007 SBM Cheng C P Liu W J Lo and Y P Lee Nucl Instrum Methods Phys Res A 467 1461 2001 2J T Jodkowski M T Rayez J C Rayez T B rces and S D b J Phys Chem A 103 3750 1999 3A Yabushita T Hama D Iida N Kawanka M Kawasaki N Watanabe M N R Ashfold and H P Loock J Chem Phys 129 044501 2008 128 31G Bravo P rez J R Alvarez Idaboy A G Jim nez and A Cruz Torres J Chem Phys 310 21
9. e g CH4 Ar O2 or CO b The main OH stretching band yields a shoulder at 3220 cm when water molecules do not accept any H atoms at the O atom lone pair but are able to donate H atoms to form hydrogen bonds e g C6H6 C2H2 Since H2O molecules which do not accommodate hydrogen atoms are able to position themselves in a complementary placement to form hydrogen bonds the 3220 cm peak is observed at a lower frequency than that of the main OH stretching band The structural disparities of the dilutant species in terms of shape and size will result in dissimilarities in 19 their interaction with pure amorphous ice These differences have been determined to increase in the order of CHy Ar O2 and co ino Go E eee e obsorborce 3500 3000 2500 2000 1500 1000 s99 frequency co om FIG 4 Infrared absorbance spectra of unannealed 10 K binary mixtures of H20 mole fraction 33 and other components 66 Adapted from Tielens et al Line intensity has been increased for clarity 20 2 2 5 Bases Acids and Amphoteric molecules Dilution of H2O with hydrogen bonding molecules alters the H2O IR absorption spectrum Consider an incident molecule B with the ability to donate electrons to create hydrogen bonds e g NH3 Such a molecule is considered to have characteristics of a base according to the Bronsted and Lewis concepts of acids and bases The IR absorption
10. f beta2 Val Text1 Text f beta4 Val Text2 Text i 0 Do While Not EOF 1 Input 1 Ekin i a i beta2 1 beta4 1 sumofa sumofa a i If Ekin i gt maxEkin Then maxEkin Ekin i i 1 Loop Close 1 numberofchannels i 1 s Numberoflons sumofa Q 96800 Counter 0 maxrho Q length Field maxEkin maxRad 1 1 2 length Sqr maxrho add 10 to the radius to create frame around image Open outputfile txt For Output As 2 135 Print 2 Numberoflons Field length For i 0 To numberofchannels rho Q length Field Ekin i For j 0 To a i s R1 RndQ R2 Rnd R3 Rnd If beta2 i 0 5 3 Cos R1 pi 2 1 1 4 pi 1 beta2 i 0 25 Sgn beta2 1 0 75 gt R2 Then theta acos R1 Counter Counter 1 phi R3 2 pi X 2 length Cos phi Sin theta Sqr rho Z 2 length Cos theta Sqr rho Print 2 X Z XI Int Z maxRad 1 res 2 YI Int X maxRad 1 res 2 IntImage XI YI IntImage XI YI 1 If IntImage XI YI gt fullscale Then fullscale IntImage XI YT End If Next j ProgressBar1 Value i numberofchannels 100 Next i Close 2 Labell Caption Done Str Counter ions 136 If Check1 Value False Then fullscale 255 Open Monte Carlo Image raw For Output As 4 For 1 0 To res 1 For j 0 To res 2 image i j Int IntImage i J 255 fullscale Write 4 ima
11. whereas a sin 0 distribution is produced if the transition dipole moment lies perpendicular to the bond A 45 single photodissociation event AB hv gt A B where mass A gt mass B creates the Newton spheres shown in Figure 8 Over time the photodissociation will produce two nested spheres with a radii ratio of Rac Ma 2 5 R Mg where the radius of each sphere is given by R A 2 6 m when f is the time of flight Event 1 Event 2 FIG 8 Nested Newton spheres photofragments A and B where ma gt mg The polar angle is defined with respect to the z axis the azimuthal angle is and the radius is given by R Reproduced from Whitaker 46 Eppink and Parker improved the original technique significantly in terms of spatial resolution when they replaced the conventional grid electrode assembly used in ion imaging with a three plate electrostatic lens with open electrodes They also introduced an additional extractor electrode to the ion optics assembly Additionally this lens system could be tuned so that the ions with the same initial velocity are mapped onto the same point on the detector regardless of their initial spatial position Hence this technique of ion lens optics and 2D imaging became known as Velocity Map Imaging and has become an essential tool in many different fields The typical imaging setup includes a pulsed molecular beam source an ion optics assembly comprised of rep
12. 0 8 kJ mol Figures 38 a and 38 b show TOF spectra for OH v 0 and OH v 1 obtained by monitoring the Ri 1 R 5 line and R 2 line respectively in the REMPI spectrum Both spectra were reproduced by a single MB distribution with Tyan 3000 500 K KEtrans gt 49 9 16 6 kJ mol In contrast to CH products the Trans 90 K component was not detected The OH REMPI spectrum in figure 39 a shows the overlapping contributions from OH v 0 and OH v 1 recorded at t 2 0 us The spectral simulation shown in figure 38 b determines the rotational temperature 7 o v 0 to be 300 100 K lt Eyor gt 2 5 0 8 kJ mol and T o v 1 to be 200 50 K KE 1 7 0 4 kJ mol as summarized in Table 4 The OH v 1 OH v 0 population ratio is 0 2 0 1 113 TABLE 4 Translational and rotational temperatures and energies of OH v 0 and 1 products OH v 0 OH 1 Translation Rotation Translation Rotation Temperature y b 3000 500 300 100 3000 500 200 50 K Energy kcal mol 49 94 8 3 2 5 0 8 49 9 8 3 1 7 0 4 Population ratio OH v 1 OH v 0 0 2 0 1 time of flight 2 0 us 114 OH G 0 arb units OH 1 Time of flight ps FIG 38 TOF spectra of OH radicals from the 157 nm photolysis of a fresh ASM sample obtained by monitoring a the R 1 Rj 5 line in the OH Ds v 0 X Trhy 0 two photon transition and b the R 2 line in the OH gt v o X
13. A 5ns C Tot 2 us D C 0 where channel A is referenced to To and channel B is set to A s delay plus 5 ns This allows the program to specify the start time and the pulse width The program allows the user to control the each channel through the Time Base Menu in the front panel and the Delay Menu The DG 535 can be triggered internally or externally The external trigger port on the DG 535 is connected to the Excimer laser Lambda Physik LPX 2001 The four pulse outputs are labeled AUB ANB CUD and COD The pulse output AAB is connected to the high voltage gate and AUB displays the same pulse on the oscilloscope The pulse output CUD is connected to the Trig in port for the CCD camera on the computer in order to control the CCD exposure time and CAD displays the same pulse on the oscilloscope The front panel of the DG535 VI is shown in Figure 22 82 SRS DG535 Four Channel Digital Delay Pulse Generator VISA resource name DG535 k TimeBase Delay s Men i a i0 pooo0 Bsli snom Cals sameo Daji nor trigger i line f m trigger slope input impedance burst s B edge iso Ohm 3 ve edge High Z single shot external internal burst trigger rate burst period burst count internal 4 t0 000 CSOC id iz FIG 22 DG 535 front panel The front panel allows the internal trigger mode to be manipulated by setting the trigger rate the external trigger mode can be controlled by spe
14. E EE E EEE RES iv Tableof Contents iaa a e Aa a a a A Aa E EAS vii List OF Fi LUTES sap es seek ots devia a ia E A A S a A oni a A E i nears x Chapter Ts rode OA a a a ees Ba shee l eLA o a atey 01e a EEE T 4 Chapter 2 Literature REVIEW snie iien ii eiaa NETAS EEEN OK EAEE EEEa ETE A 5 2 0 INPOCUCHON sae eristaatus aa dhe tease EE E r e E 5 2 1 Structure and Formation of water iCe cccccecce cee ce ence ee eee eee e nena eee eae ene ene eas 6 2 2 Spectroscopic studies of vapor deposited ice filMs 6 0 cece eee e eens eee eeeenees 11 2 2 1 P re HO seoor erodar or eE RE A oe i E ARERO iht 11 2 229 Dangling OF seszinnsccasncsaaigags os dened acces e E E E A wehbe tbs Lh ane anes 14 2 2 2 1 Temperature and pressure dependence of the dangling OH bond 68 17 2 2 3 Interaction with other species 00 ccc cceecece ence cee nent eee eee eee teen tent ene ena ees 18 2 2 4 Species with incomplete hydrogen bonding cccceceeceeceee ene eneeneennees 19 2 2 5 Bases Acids and Amphoteric molecules ccecceceeceene ences eee eeee ene ene ees 21 2 3 Amorphous Solid Waters iccs0c nerie a o a ou ave cnet ta ee eked cua ese ed cadena oes 23 2 3 1 Vapor deposition methods oeseri ceriti i cece cee eee e nen EE AEA U ia EEE 23 2 3 2 Micropore Seanina a ae a n Meds ach hdoale ae baceenesiahte siebaneaetes 29 2 3 3 Trapping of paS orai ous iads head E eb idaeat end dayste Setdeaneadeuweniaeteade
15. The gaseous sample is then frozen on to a sapphire rod Cryo industries supported in the middle of the vacuum chamber The sapphire rod is attached to a helium compressor Sumitomo Cryo Industries HC 4E and has an Au 111 plated tip The temperature of the sapphire rod is controlled in the range 8 120 K by a temperature controller and a sensor in the cold finger assembly Photofragments are produced from the frozen gas sample by laser photolysis of the ice matrix They are detected through the 2 1 REMPI process using a second delayed probe laser The probe laser is a tunable and frequency doubled dye laser Lambda Physik Scanmate 2E Dye Laser that is pumped by a XeCl Excimer Laser Lambda Physik LPX 200 160 200 mJ pulse operating at 308 nm The output beam from the dye laser in the probe laser system is frequency doubled using B BaB2O crystal Photolysis can be performed using either another dye laser Lambda Physik Scanmate 2E pumped by an Nd YAG laser Continuum Powerlite Precision 8000 532 nm or by using an Optical Parametric Oscillator OPO unit pumped by an Nd YAG laser Spectra Physics 51 355 nm The output of the OPO unit can be tuned between 214 354 nm The laser beams are perpendicular to the TOF axis and meet the molecular beam between the repeller and extractor electrodes Infrared spectra will be collected before and throughout the photolysis of the frozen gas sample with a commercial FT IR spectrometer Th
16. Upon my return I was able to perform simulations of the photofragments using PGOPHER software by C M Western University of Bristol with the help of postdoctoral fellow Wei Guo This chapter is based on the experiments performed in Kyoto The figures used in the experimental and results sections in this chapter have been reproduced from a publication co authored by my collaborators and myself 5 1 Introduction Interstellar dust grains with ice mantles coating their surface are rich with numerous chemical compounds such as H20 NH3 CH4 O2 N2 CO CO2 and H2CO As interstellar dust grains are continuously exposed to a low flux of UV photons molecular bonds are broken within femtoseconds and the photofragments recombine giving rise to complex 100 molecules The production pathways of such molecules have been of great astrophysical interest 4 Methanol is also known to be a primordial constituent found abundantly on the icy mantle of dust grains comets and other solar bodies Mumma et al reported that the abundance of methanol in Oort cloud comets is 1 3 relative to water The photochemistry of the methanol rich interstellar medium is expected to form large organic molecules Various dissociation mechanisms at 157 nm for methanol in the gas phase have been reported Lee et al suggested that the detected photoproducts were formed through the channels given in reactions 3 1 3 5 and that secondary dissociat
17. and the collapse of micropores during annealing at 120 K The collapse of the micropores accompanies the densification and a change in the structural organization of the water ice network The temperature dependence of the dangling OH bond signifies that there are two regions for structural transitions in vapor deposited ice that corresponds approximately to the regions where the density changes rapidly Hence the spectral feature of the dangling OH bond can also be used as a means to investigate the surface properties of water ice 17 In the ballistic deposition model of ice the incident H2O molecules are deposited on the ice surface with a sticking probability of one as they encounter unoccupied sites H20 desorption is negligible at temperatures below 140 K Hence an incident H2O molecule brought to the water ice surface will be either buried by a successive inward bound H2O molecule or may diffuse to an unoccupied site in the ice multilayer The time available for the admolecules to diffuse before they are buried by the H2O layer is dependent on the incident H O flux At a high vapour deposition pressure molecules are buried faster than they can diffuse As the temperature decreases at a fixed deposition rate it is less likely that a given molecule will have sufficient energy to the find a more favourable site in the network If the molecule is buried in a higher orientation more micropores and dangling bonds result within the
18. are ionized and detected Since the initial kinetic energy of the desorbed products are estimated to be less than 1 eV and since the kinetic energy gained through the accelerating field is extremely large the equations for positions y z are reduced to _ 2Lcos sin0 ap 2 13 61 _ 2Lcos T The fragments from photodissociation are ejected parallel or perpendicular to the Z 2 14 polarization direction of the light with an angular distribution given by I 0 1 8 P cos 47 2 15 where is the angle between the fragment velocity vector and light polarization direction and P x 3x 1 2is the second order Legendre polynomial The y z co ordinates are calculated by the Visual Basic program as stated below The program generates three random numbers R R2 and R3 which are distributed evenly between zero and one From R one can calculate 27R However is not calculated directly as 0 zR in the program since even for an isotropic distribution this equation would bias towards intensity at the poles of the image For an anisotropic distribution the intensity is given by equation 2 15 Using a random number generator it is possible to construct random numbers taken from a variety of distributions If there are equal distributions of random numbers R Rz and R the random number generator can be used to generate the numbers If the random numbers are of a linear distribution
19. be possible to simulate the water ice matrices in polar regions of the earth containing contaminants such as nitrates and sulfates using the VMI apparatus 132 Appendix Private Sub cmdReadFile_Click Dim Ekin 500 a 500 beta2 500 beta4 500 pi 3 141592 sumofa 0 Dim R2 As Double ChDir c Data Piyumie Monte Carlo Image Creation Change this path to where ever you want you work environment to be Open inputfile txt For Input As 1 Input 1 Numberoflons Input 1 Field Input 1 length i 0 Do While Not EOF 1 Input 1 Ekin i a i beta2 i beta4 1 sumofa sumofa a i i i 1 Loop Close 1 txtNumberoflons Text Numberoflons Text Text beta2 Text2 Text beta4 numberofchannels i 1 s Numberoflons sumofa Q 96800 133 Counter 0 End Sub Private Function acos y As Double Dim v As Double v y 2 1 acos Atn v Sqr v v 1 2 Atn 1 End Function Private Sub Command1 Click Dim Ekin 500 a 500 beta2 500 beta4 500 Dim IntImage 1050 1050 As Integer Dim image 1050 1050 As Byte Dim R2 R1 R3 As Double pi 3 141592 sumofa 0 maxEkin 0 res Val Text3 Text For i 0 To res For j 0 To res image i j 0 IntImage i j 0 Next j Next i ChDir c Data Piyumie Monte Carlo Image Creation 134 Open inputfile txt For Input As 1 Input 1 dummy Input 1 Field Input 1 length Numberoflons Val txtNumberoflons Text
20. constituents 69 References for Chapter 3 B J Whitaker Imaging in Molecular Dynamics Cambridge University Press Cambridge 2003 B J Whitaker Image Reconstruction The Abel Transform in A G Suits R E Continetti Eds ACS Symposium Series Vol 68 Oxford University Press 2000 A Heck Europ Mass Sectr 3 171 1997 P L Houston Acc Chem Res 28 458 1995 P L Houston J Phys Chem 100 12757 1996 S H Lee H I Lee and Y T Lee J Chem Phys 121 11053 2004 7J Solomon J Chem Phys 47 889 1967 D W Chandler and P L Houstan J Chem Phys 87 1445 1987 W C Wiley I H McLaren Rev Sci Instrum 26 1150 1955 1A J R Heck and D W Chandler Annu Rev Phys Chem 46 335 1995 1C Vallance Philos Trans R Soc London A 362 2591 2004 2D M Sonnefroh and K Liu Chem Phys Lett 176 183 1991 3A T J B Eppink and D H Parker Rev Sci Instrum 68 9 1997 B Y Chang R C Hoetzlein J A Mueller J D Geiser and P L Houston Rev Sci Instrum 69 1665 1998 I5 S Bracker E R Wouters A G Suits and O S Vasyutinskii J Chem Phys 110 14 1999 16G A Gracia L Nahon and I Powis Rev Sci Instrum 75 4989 2004 70 17C Bordas F Pauling H Helm and D L Huestis Rev Sci Instrum 67 6 1996 8M J Bass M Brouard A P Clark and C Vallence J Chem Phys 117 19 2002 IM
21. et al reported that a significant amount of radiation penetrates through relatively thin molecular clouds since their inception As a result of photoerosion volatile molecules cannot remain stable at the surface of the molecular cloud Hence dust is found as bare silicate or carbonaceous particles in the molecular cloud The temperature of the molecular cloud begins to lessen as the density of the dust particles increases This process is followed by a reduction in the photon field due to optical absorption A presolar molecular cloud is formed when a molecular cloud is cooled from 1000 K to 10 100 K Due to the exceeding reduction in the photon field atoms and molecules deposit on the dust surfaces and can now form a solid ice mantle At the end of this process a presolar molecular cloud contains dust consisting of core mantle particles and an additional outer mantle of volatile ices dominated by H20 as given in Figure7 0 5 um FIG 7 Structure of a cosmic dust particle Adapted from Herbst 33 Over 120 molecular species of ions and complex organic molecules have been detected in molecular clouds These species evolve from atoms and other simple molecules through surface mediated chemical reactions Almost 50 percent of the species found in molecular clouds are familiar terrestrial species such as water ammonia formaldehyde 1 33 and simple alcohols such as methanol and ethanol The other 50 perce
22. excitation spectrum of CH radicals resonance enhanced at the two photon energy by the 4p A v 0 level assuming a Boltzmann distribution rotational state population distribution with To 150 K 111 TABLE 3 Translational and rotational temperatures and energies of CH v 0 products Translational Rotational Translational Time of flight component energy temperature energy contributions Ex kJ mol T K Er CH Trans 3000 K 10 49 9 42 0 E j CH Trans 90 K 90 1 50 3 150 50 1 2 0 4 The rotational spectrum could not be characterized by a specific T o because of the weakness of the signal intensity From spectra recorded at t 6 0 4s CH signal intensity arb unit Time of flight ps FIG 36 TOF spectrum of CH radicals from the 157nm photolysis of a fresh ASM sample monitoring on the Q branch of the 2 1 REMPI spectrum shown in Fig 35 The solid curves are fits to the data derived assuming two MB distributions with Tyan 3000 K 10 and 90 K 90 The vertical flight distance used in these experiments is 2 mm 112 5 4 2 Kinetic energy and rotational distribution of the OH radical Figure 37 a shows the REMPI rotational spectrum of OH v 0 products following the 157 nm photolysis of freshly deposited ASM recorded at a fixed delay of t 2 0 us The spectral simulation shown in figure 37 b determines the rotational temperature T ot v 0 to be 300 100 K lt Eyo gt 2 5
23. from the velocity broadened distribution used for the simulation 1 8 1 0 0 5 0 5 3 4 Conclusion B recovered from the Onion Peeling Program 1 77 1 03 0 08 0 5 0 51 B recovered from the Abel transform 1 82 0 94 0 1 0 5 0 52 A significant amount of information about the ice matrix and the desorbed species from the ice surface can be extracted by combining the REMPI spectroscopy and FT IR spectroscopy In this section the principle of the velocity map imaging spectrometer has been discussed together with the inversion procedure to extract physical information from the acquired images Resolution characteristics of the velocity map imaging spectrometer have been investigated with the ion trajectories simulation using Simion 7 0 software package The results from this simulation provided an understanding of the voltages that can be applied to the ion optics assembly to optimize the resolution of the 68 experimental image of photoproducts With the Visual Basic program discussed in this section it is possible to simulate the images of the desorbed species from the ice surface However the expected image would be a semi circle as desorbed species are only directed to the space above the cold tip These simulations will aid in acquiring the velocity profile of the photoproducts and provide more accurate energy distributions to propose mechanisms for reactions in water ice and its
24. ice bulk 2 2 3 Interaction with other species Tielens et al conducted an extensive study on the interaction of amorphous ice with other species The spectroscopic properties of water ice undergo changes upon interaction with impinging molecular species Adsorbent species may alter the structure of amorphous solid water based on factors such as their size shape and hydrogen bonding ability Due to the existence of impurities the degree of intermolecular coupling becomes smaller and the hydrogen bonds become weaker because of increased O O separation This phenomenon leads to an increase in the OH stretching frequency and a decrease in librational and bending frequencies In addition to these changes the OH stretching band 18 was reported to have become less intense as a result of the reduction in induced polarization 2 2 4 Species with incomplete hydrogen bonding Molecules that are incident on the ice surface and that show deficiencies in forming hydrogen bonds can be identified by distinguishing features observed in the IR 9 17 absorption spectra The following features can be seen as a result of the incomplete hydrogen bonding in the H2O network as given in Figure 4 a The terminal OH group in the water molecule can be identified by a distinct OH stretching frequency around 3700 cm in the presence of dilutant molecules that are incapable of accommodating hydrogen atoms to form hydrogen bonds
25. into a computer through a connector block CB 68LP NI PCI 84 counter timer devices can be easily used across multiple hardware devices as they are equipped with a Real Time System Integration RTSI bus line 4 3 2 Triggering with PCI 6602E VI This VI has been written using the DAQ features in order to synchronize instrument components in the experiment such as the Nd YAG laser Delay Generator and Pulsed Valve The synchronization process is performed by the Master Device and Slave Device panels seen in the front panel of the VI Hardware synchronization is achieved through the RTSI cable which is a high speed digital bus that exchanges timing signals between NI boards In the LabVIEW programming environment the NI DAQ driver is responsible for routing and driving the RTSI bus automatically via the Route Signal VI The LabVIEW program written for the PCI 6602 makes use of this feature by generating a pulse in counter 0 and then translating that pulse to all the counters through the RTSI bus thereby synchronizing the connected devices The pulse for the master device was programmed by using the classes of the DAQ Create Virtual Channel The numerous classes of the DAQ Create Virtual Channel are able to perform functions such as temperature measurement voltage generation event counting and corresponding with I O channels The instance of this channel employed in programming PCI 6602 is CO Pulse Time The C
26. make no contribution to their formation REMPI signals for O D and OCP atoms were not observed in the TOF range at 0 5 us lt t lt 30 us Hama et al have previously detected O D and O P following the 157 nm photodissociation of H2O and H202 The lack of detectable O D and OCP atoms from ASM is in agreement with methanol photolysis in the gas phase reported by Hariach et al and Lee etal They reported that formation of methane and O D or OCP atoms through reactions 3 14 and 3 18 is insignificant in the photolysis of methanol CH 0H hvu CH 0 D 3 14 CH3 30H hv CH OCP 3 18 In order to ensure that the CH distribution was not effected by the background water vapor deposition the TOF signal was measured from a photoirradiated 1 1 mixture of 117 CH30H H20 ice The TOF intensities for CH3 in both components at Trans 3000 and 90 K decreased Additionally similar to the fresh ASM sample a majority of the CH3 fragments are accommodated to the substrate temperature of 90 K The effects on the OH distribution could not be determined in a similar manner as the photolysis of H2O contributes to the OH distribution 5 5 Discussion The photodissociation of methanol in the condensed phase through the C O bond cleavage channel is given in reaction 3 19 where ads is the condensed or the adsorbed species and i is the ASM vacuum interface CH3OH ads hv gt CH3 i OH i A
27. of the substrate the incidence angle and the flux Therefore it is necessary to be very specific about the ice matrix created in experimental studies The morphology of the water ice matrix can be characterized by FT IR spectroscopy as described in section 222 The experimental studies conducted in Kyoto University on the 157 nm photolysis of the methanol ice matrix probed by 2 1 REMPI demonstrates that many secondary photo processes take place in the ice matrix Using the apparatus under development at Queen s University a significant amount of information about the ice matrix and the primary and 130 secondary photoproducts from the ice surface can be extracted through the combination of the REMPI spectroscopy and FT IR spectroscopy These studies will be valuable in illustrating the structural changes that take place in the ice matrix and will allow the reaction mechanisms for the secondary photoprocesses to be explored comprehensively The addition of VMI spectroscopy expands this investigation further by providing information about kinetic and internal energy distributions of the desorbed species and the dynamics taking place during photodissociation The present work involves the design and development of the VMI apparatus together with the development of software components required for experimental studies Resolution characteristics of the apparatus were studied with ion trajectories simulations using the Simion 7 0 software pack
28. reactions have been studied over a long perio the insight that the profusion of some molecules such as hydrogen molecules cannot be explained by pure gas phase formation has drawn considerable attention to the surface reactions on dust grains which account for the formation of these molecules The cosmic ray ionization of hydrogen molecules produced on grains and desorbed into the interstellar gas is indicated below Hz cosmic ray gt H e cosmic ray 1 1 Hv ion reacts with molecular hydrogen to produce H3 H H gt H H 1 2 The H3 ion is a comparatively abundant species as it does not react with molecular hydrogen Hence the H3 ion serves as a precursor for gas phase reactions in the molecular cloud with its ability to react with other species For example the reactions with atomic oxygen initiate a chain of reactions leading to the production of the hydronium ion via reactions 1 3 1 4 and 1 5 H O gt OH H 1 3 OH H gt HO H 1 4 H2O H gt H30 H 1 5 35 Besides gas phase reactions in the cold region of the molecular clouds where dust grains are covered with the water ice mantle H production through the photolysis of H2O molecules in icy mantles is important Yabushita et a measured kinetic energy and the rovibrational population of H2 molecules produced by 157 nm photolysis of water ice at 100 K using the REMPI method When water ice is exposed t
29. surfaces and the states of the adsorbed species have not been studied simultaneously The combination of velocity map imaging to determine the kinetic energy and the angular distribution of the photoproducts and FT IR spectroscopy to determine the chemical composition of the ice matrix before and throughout photolysis provide an optimized method to investigate the photochemistry of water ice and its constituents The characteristics of the velocity map imaging spectrometer are important when acquiring the velocity profile of a photofragment In the following section the characteristics of the velocity map imaging spectrometer are discussed through a simulation using the Simion 7 0 software package The dependence of the resolution on the ion optics voltage is explored through this simulation Additionally a software program written using Microsoft Visual Basic 6 0 to simulate the experimental pattern observed on the position sensitive detector from the photoproducts is discussed in detail These simulations provide useful information for estimates of resolution for experiments with other photoproducts 54 3 2 SIMION Simulations The mapping of the 3D distributions of the desorbed photofragments onto the 2D detector plane is dependent on the configuration of the electrostatic lens system in the TOF and the voltages applied to them The TOF consists of three electrodes namely repeller R extractor E and ground G These are used to acc
30. systems Springer 2007 http www ni com LabVIEW help menu Stanford Research Systems Series PS 300 User Manual 1998 Stanford Research Systems DG 525 User Manual 1999 National Instruments DAQ 6023E 6024E 6025E User Manual January 1999 MS03000 and DPO3000 Series Digital Phosphor Oscilloscopes User Manual Tektronix 8D M Bolodovsky Photodissociation dynamics of bromine chloride probed by velocity map imaging M Sc thesis Queen s University 2006 C Romanescu Velocity imaging of the photodissociation and photoionization dynamics of hydrogen chloride and hydrogen bromide Ph D thesis Queen s University 2007 C Elliott V Vijayakumar W Zink and R Hansen National Instrument LabVIEW A programming environment for laboratory automation and measurement The association for Laboratory Automation 2007 SensiCam Optikon Operating Instructions 1999 121 abVIEW drivers for Sensicam Version 3 0 99 Chapter 5 Photolysis of Amorphous Solid Methanol at 157 nm 5 0 Background During the period from November 2008 December 2008 I had the opportunity to work with Prof Masahiro Kawasaki s research group in Kyoto University Japan under an Ontario Centre for Excellence fellowship During this period I became involved in experiments on the photochemistry of methanol ice as part of the preliminary experiments required for the development of the VMI apparatus at Queen s University
31. the 157 nm photolysis of a fresh ASM sample monitoring on the Q branch of the 2 1 REMPI spectrum shown in Fig 35 112 Figure 37 a 2 1 REMPI excitation spectrum of OH radicals from the 157 nm photolysis of a fresh ASM sample at 90 K recorded at t 2 0 us b Simulation of the Ds v 0 xX Trhy 0 two photon excitation spectrum of OH assuming a Boltzmann rotational state population distribution with To 300 K 114 Figure 38 TOF spectra of OH radicals from the 157 nm photolysis of a fresh ASM sample obtained by monitoring a the R 1 R 5 line in the OH D gt v O e X Trhy 0 two photon transition and b the R 2 line in the OH 37x v 0 xX Thy 1 two photon transition 115 Figure 39 a 2 1 REMPI excitation spectrum of OH radicals from the 157 nm photolysis of a fresh ASM sample at 90 K recorded at t 2 0 uss b Simulation of relevant parts of the overlapping D v le X Trhy 0 and 3 hv O e xem 1 two photon transitions of OH 116 xii List of Tables Table 1 Results for the radius of the image and magnification factor based different voltages applied to the electrodes to ions with 15 amu mass and 1 eV of kinetic energy Table 2 Anisotropy parameters for simulated image and inverted image from the velocity broadened distribution
32. they can be generated by the von Neumann algorithm The von Neumann algorithm generates non linearly distributed random numbers using a biased condition statement as follows 62 This algorithm generates an even distribution of random numbers in the range x1 x3 To generate a random number R uniformly on x x2 assuming we can generate randomly on the range x1 x3 i a er a XxX X2 X3 To generate a linearly distributed array of random numbers the algorithm is modified to a Generate R uniformly on x x3 and generate Ry uniformly on x1 x3 b If Rj lt R then take R as the random number on the distribution c If not go to a The expression for 0 is still more complicated and given in 2 15 We can then generate the random angular distribution in 0 from a generate R uniformly on x x3 and generate Rz uniformly on x1 x3 b if B x 0 5 x 3 x Cos R x m 1 1 lt R then take R as the random number on the distribution The angle 6 arcos R2 c if not go to a If the value obtained from the conditional statement is greater than R2 then the value is kept without being discarded The Visual Basic program therefore contained the following code 63 If B x 0 5 x 3 x Cos Ri x m 2 _ 1 1 1 B x 0 25 Sgn p x 0 75 gt Ro Then 0 arccos R 2R y 2LCos Sin0 T Z 2LCos 0 Je The expression J x 0 25 Sgn f x 0 75 is a normalizati
33. upon the initial expansion speed and the time of flight 57 FIG 14 The trajectories originate at y 0 mm with one ion ejected with a 90 elevation angle and other at 0 elevation angle The empirical relation between magnification factor N expansion speed v time of flight and the radius R of the ring appearing on the image is described by R Nvt 2 7 Two ion trajectories originating at the same point with elevation angles of 0 and 90 were projected onto the detector and the radius of the ring was obtained With this parameter the magnification factor of the image was calculated to be 1 27 The radius of the image obtained for different voltages applied to the electrodes are given in Table 1 58 TABLE 1 Results for the radius of the image and magnification factor based on different voltages applied to the electrodes using ions with 15 amu mass and 1 eV of kinetic energy 500 275 37 58 33 1 27 750 413 06 41 51 1 27 1000 550 75 36 09 1 27 1250 688 43 32 33 1 28 1500 826 12 29 51 1 28 3 3 Monte Carlo Image Creation To simulate an image that is close to the image expected from experiments a software program was written in Microsoft Visual Basic 6 0 An 8 bit 512x512 pixel image was obtained by simulating 1 000 000 trajectories with the extractor electrode voltage calculated from the SIMION simulation kinetic energy of ions amplitude of channels magnification factor of the image and the anisotrop
34. 3 2005 32A Yabushita T Hama D Iida and M Kawasaki J Chem Phys 129 014709 2008 3T Hama A Yabushita M Yokoyama M Kawasaki and S Andersson J Chem Phys 131 054508 2009 g Andersson A Al Halabi G J Kroes and E F van Dishoeck J Chem Phys 124 064715 2006 35S Andersson and E F van Dishoeck Astron Astrophys 491 907 2008 M C Akin N G Petrik and G A Kimmel J Chem Phys 130 104710 2009 37A W Jasper S J Klippenstein L B Harding and B Ruscic J Phys Chem A 111 3932 2007 PR Hodyss P V Johnson J V Stern J D Goguen and I Kanik Icarus 200 338 2009 S C Schwentner R M Schriever and M Chegui J Chem Phys 95 6124 1991 TH Piette R C Rempel and H E Weaver J Chem Phys 30 1623 1959 129 Chapter 6 Summary 6 0 Summary In the interstellar medium and in the polar regions of earth water ice is a medium for chemical reactions when exposed to ultraviolet radiation Amorphous solid water not only has fascinating photochemistry leading to the formation of photoproducts such as H2 OH H20 but it also can change the photochemistry of adsorbed species In experimental studies it has been found that the photo initiated reaction mechanisms between water ice and adsorbed species are dependent on the morphology of water ice The morphology of water ice may be changed by modifying deposition conditions such as the temperature
35. 34 the flight length is defined by e R y and the detection probability is proportional to 27zROR e R where r 2 and 4 mm for CH and OH respectively and 0 lt R lt 6 mm The variable R is the radius of the irradiation area For the angular distribution of the photofragments from the ice surface cos 0 where 0 is the polar coordinate n 0 was assumed in the best fitting procedures as the parent CHOH molecules adsorb randomly on the ASM surface 108 Detection point FIG 34 Detection of the photofragment from the surface A circle indicates the irradiation area on the substrate is the distance between substrate and the detection point R is the maximum radius of the irradiation area Reproduced from Yabushita et al 5 3 Simulation of 2 1 REMPI spectra The 2 1 REMPI spectra for the 4p 74 v 0 xX 4 v 0 transition of CH products following was simulated by the Pgopher software package by using the spectral parameters reported by Black and Powis The 4p 74 v 0 state 109 predissociates with a level dependent efficiency and this phenomenon affects REMPI line intensities and line width The two photon transition probability is carried by two components ranked zero and two The zero component contributes only to the intense central Q branch while the component ranked two contributes to O P Q R and S branches Similarly the simulation for OH products was performed using the sp
36. 8 2 3 2 Micropores Vapor deposited ice at 12 K grows with an extremely uneven surface that results in a microporous network with 5 10 of the molecules on the ice surface When water molecules incident on the ice surface have inadequate time to move to a suitable site before being covered by subsequent adlayers they are buried in random orientations and leave gaps within the bulk ice As a result microporous networks are formed in the ice film When these gaps are of a sufficient width they prevent the formation of hydrogen bonds across the pore Hence 3 coordinate OH groups that are non hydrogen bonding are formed at the pore surface Devlin and Buch observed the loss of surface OH vibrational signature between temperatures of 30 60 K They proposed that the weakening of the IR absorption signal for dangling OH groups was a consequence of the solid water structure becoming less dense as the water molecules achieve a fuller co ordination This process yields an open tetrahedral network Zondlo et al investigated the behavior of dangling OH bonds in vapor deposited ice between 90 and 120 K and reported that the intensity of the dangling OH feature increased linearly with the thickness of the ice film They proposed that the majority of the dangling OH was formed on the micropores of ice inside the condensed ice bulk In addition they suggested that low substrate temperatures and fast deposition rates led to the profusion o
37. E Lyke R D Campbell J Aycock A Conrad and G M Hill Science 310 270 2005 S Harich J J Lin Y T Lee and X Yang J Chem Phys 111 5 1999 1S Harich J J Lin Y T Lee and X Yang J Chem Phys 103 10324 1999 85S H Lee H I Lee and Y T Lee J Chem Phys 121 11053 2004 8 Satyapal J Park R Bersohn and B J Katz J Chem Phys 91 6873 1989 T Hama M Yokoyama A Yabushita and M Kawasaki J Chem Phys 30 164505 2009 IIA Yabushita Y Inoue T Senga M Kawasaki and S Sato J Phys Chem B 106 3151 2002 2A Yabushita Y Hashikawa A Ikeda M Kawasaki and H Tachikawa J Chem Phys 120 5463 2004 BS M Dounce J Mundy and H L Dai J Chem Phys 126 191111 2007 4J F Black and I Powis J Chem Phys 89 3986 1988 ISM E Greenslade M I Lester D C Radenovic A J A van Roij and D H Parker J Chem Phys 123 074309 2005 16C M Western PGOPHER a program for simulating rotational structure University of Bristol available at http pgopher chm bris ac uk 127 1K P Huber and G Herzberg data prepared by J W Gallagher and R D Johnson III Constants of Diatomic Molecules in NIST Chemistry WebBook NIST Standard Reference Database Number 69 P J Linstrom and W G Mallard National Institute of Standards and Technology Gaithersburg MD 20899 see http webbook nist gov retrieved 2009 BS T Pratt P
38. H 422 6 kJ mol 3 19 The photonic energy provided for the reaction at 157 nm is 757 3 kJ mol and the energy available for this reaction has been calculated as 334 7 kJ mol 78 In the gas phase photolysis of CH3OH at 157 nm Lee et al and Harich et al have shown that the distribution of the total kinetic energy for CH and OH is 217 6 kJ mol The translational kinetic energy partitioned between CH3 and OH according to the conservation of momentum is calculated respectively as 117 2 and 100 4 kJ mol 118 In the condensed phase the average center of mass translational energy for the photodesorbed CH and OH products is only 99 8 kJ mol This reduction in partitioning of E avail 3 19 could be due to the dissipation of energy into the ASM bulk 5 5 1 CH radical formation from the photolysis of fresh ASM The TOF spectrum of CH products measured at 157 nm was attributed to the sum of two MB components They are distinguished as a high temperature fast component at Trans 3000 K and a surface accommodated slow component at Tirans 90 K with respective contributions of 10 and 90 A majority of the CH fragments are accommodated to the substrate temperature of 90 K and as given in Table I the translational and rotational energies are considerably lower than Eavai 19 Hence the dominance of the surface accommodated component is accredited to nascent CH3 products formed in the ASM bulk losing a substantial
39. N Kawanka M Kawasaki P D Hamer and D E Shallcross J Chem Phys 129 014709 2008 4S Malyk G Kumi H Reisler and C Wittig J Phys Chem A 111 13365 2007 42 Chapter 3 Velocity Map Imaging and Simulations 3 0 Introduction The technique of ion and electron imaging has become an indispensible tool in the study of chemical dynamic processes such as bimolecular reactions photodissociation and photoionization Imaging techniques allow three dimensional 3D angular and velocity distributions of the products formed from photolysis to be visualized directly In this chapter the developments in velocity map imaging will be reviewed the characteristics of the velocity map imaging spectrometer will be discussed using the Simion 7 0 software package and the simulation of the experimental image of ionized photoproducts will be illustrated using Microsoft Visual Basic 6 0 The earliest study on Photolysis Mapping was reported by Solomon in 1967 He investigated the photolysis of bromine and iodine molecules in a glass hemisphere coated with tellurium with polarized light emitted from a mercury lamp Upon photolysis the halogen photofragments removed the tellurium in the photolysis cell and created an anisotropic depletion pattern This first visualization of the spatial distribution of photofragments led to the evolution of ion imaging techniques 43 Chandler and Houston marked a phenomenal advancement in
40. O Pulse Time Channel creates a digital pulse defined by the pulse width and initial delay specified by the user The initial 85 state of the pulse was defined as low The basic implementation of DAQ features to generate a pulse is shown in Figure 23 Counter Output Event F DAQmx Is Task Done vi Connection Use a RTSI Line FIG 23 DAQ in the PCI 6602E VI 4 3 2 1 Implementation of the PCI 6602E VI The input task in is then provided to the Timing Implicit VI which in turn configures the duration of the pulse generation and the number of pulses to generate The Export Signal VI then routes the control signal to the terminal specified by the user The Start VI initiates pulse generation and the Zs Task Done VI queries the status of the task and indicates the completion of the execution cycle Once the execution cycle has been 86 completed the Clear Task VI stops the task releases the resources reserved and clears the task Similarly the digital pulses for all the slave devices are generated by the CO Pulse Time Channel The I O counter denotes the names of the counters to create the virtual channels The DAQmx physical channel constant provides all the counters and devices installed in the system and they can be selected from the drop down menu in the front panel 4 3 2 2 Front Panel of the PCI 6602E VI The front panel is comprised of two main panels for master and sla
41. T EE E ee T 84 4 3 2 Triggering with PCI 6602E ssssssssunrsssseresssssrererrsssrrresssssrererresserreeses 85 4 3 2 1 Implementation of the PCI 6602E V1 cc ccc ccc ccc cece cence cence eee e nen anette 86 4 3 2 2 Front Panel of the PCT 6602E V1 0 2 ccc ccc cc cence ene nce e ence ene nee ne ne eeeneee esta ees 87 4 4 Collecting TOF and REMPI spectra using LabVIEW ccccec eee ene ene eneenees 89 4 4 1 Stepper motor panel to control probe laser 0 cece cee ce ence nee ee eee een e ena eenaes 90 4 4 1 1 Implementation of the stepper motor panel ccc ccc e cece nce e cee cene ene eneennees 91 4 4 1 2 Front panel of the stepper motor controller 0 ccc cee ce eee e cence ene ee ene ene ee enes 91 4 4 2 Front panel of the oscilloscope c cece ence nee ne nee n eee eee ene ne ene ene ena engaaenes 92 4 4 2 1 Implementation of the oscilloscope cecc cece cence cence neces ene ene ene ea een easeesies 93 4 4 3 TOF and REMPI spectra panels 0 cece cence eee cence eee e ce eee eaten ene ene nae errs 94 4 5 Masine NA ie tees in nce it nate danrth a a tela Decstacbate dis a ERTA 94 4 5 1 Implementation of the Imaging VI 0 00 cece cece nce ee nent nents 96 4 6 Conclusion eaa a eed gana bE basal sae a a ee Roadie nee ie pose ote 98 References for Chapter 4 00 ccc cece cece cence ene een een enn ene n EEE EE ene eats 99 Chapter 5 Photolysis of Amo
42. The MCP is gated with a high voltage pulse to allow only ions of interest to be detected The TOF profile of these ions is observed through a digital oscilloscope Tektronix DPO3034 series and 76 recorded as a function of the probe laser wavelength via the Stepper Motor Scope REMPI VI The cascade of electrons that strike the phosphor screen creates an ion image which is recorded by a CCD camera Sensicam Optikon 12 bit 1280 x 1024 resolution controlled by the Imaging VI Figure 19 shows the layout of the VIs for data acquisition 77 PCI 6602E Oscilloscope Board Fig 19 Schematic for the data acquisition system using V s 78 4 1 PS 350 series High Voltage Power Supply The four PS 350 series High Voltage Power Supplies are used to control the voltages applied to the repeller plate back MCP extractor plate and the phosphor screen The PS 350 series High Voltage Power Supply is remotely manipulated by the Lab VIEW program via the GPIB interface In order to communicate with the GPIB interface the address of the PS 350 device must be configured This is achieved by selecting the appropriate GPIB name from the VISA resource drop down menu The VISA resource name is unique to an instrument I O session It specifies which device to communicate with and all the configuration information to perform the I O This prompts the opening of a session with the selected device specified by the VISA resource name and the progra
43. Thy 1 two photon transition The solid curves are fits to the data derived assuming a MB translational energy distribution with Tyan 3000 K in both cases The vertical flight distance used in these experiments is 4 mm 115 OH v 0 1 signal intensity 237 50 237 55 237 60 237 65 Wavelength nm FIG 39 a 2 1 REMPI excitation spectrum of OH radicals from the 157 nm photolysis of a fresh ASM sample at 90 K recorded at t 2 0 us b Simulation of relevant parts of the overlapping DX v ges xem 0 and 3 hv 0 xen 1 two photon transitions of OH assuming Boltzmann rotational state population distribution with T v 0 300 K blue line and Tro v 1 200 K red line The black line is the sum of the spectral simulations The arrow indicates the R 2 line used when measuring the TOF spectrum 116 5 4 3 Additional 157 nm photolysis experiments on ASM The effect of the secondary photoprocesses on the ASM surface and in the bulk on the desorbing CH and OH were assessed by measuring the TOF spectra of these species after 30 min of photoirradiation without intermissive injection of CH30H vapor into the chamber No disparities in TOF or REMPI spectra were observed between the CH or OH products for fresh or photoirradiated ASM samples This implies that the desorbed CH and OH species are formed through the C O cleavage channel and that the secondary photoprocesses on the ASM surface and in the bulk
44. VELOCITY MAP IMAGING APPARATUS FOR STUDIES ON THE PHOTOCHEMISTRY OF WATER ICE by Pubudu Piyumie Wickramasinghe A thesis submitted to the Department of Chemistry In conformity with the requirements for the degree of Master of Science Queen s University Kingston Ontario Canada February 2011 Copyright Piyumie Wickramasinghe 2011 Abstract This work describes the design and development of a velocity map imaging apparatus that will be used to study the laser initiated photochemistry of water ice and other condensed phases Experiments on methanol ice photolysis using a different apparatus at Kyoto University are described to give an appreciation of the photochemistry and the experimental parameters Water deposited on a surface at temperatures below 140 K can form an amorphous solid Amorphous solid water ASW which does not exhibit properties of a well defined phase is the most profuse phase of water found in astrophysical environments Chemical characteristics of ASW in particular its photochemistry and the physical characteristics closely associated with the structure such as density and surface are reviewed The correlations between the morphology and the growth conditions of ASW are also described Methanol is also known to be a component on the icy mantle on interstellar grains The effects of irradiating amorphous solid methanol by UV photons are discussed Experiments at Kyoto University have been perform
45. a result of microporosity can reach several hundred m7 g for ice grown at temperatures below 100 K Thus a fresh ASW surface is analogous to a high capacity vacuum pump As Baragiola notes this pumping ability and long exposure times mean that icy surfaces in the solar system may be saturated with atmospheric gases Unfortunately the high porosity and strongly hydrogen bonded surface also means that in the case of experiments contamination with background gases will be difficult to avoid 2 3 1 Vapor deposition methods A fundamental problem in studies of vapor deposited ice has been that widely varying values have been reported for physical properties of ice such as density porosity thermal conductivity and effective area for gas absorption Stevenson et al suggested that these discrepancies in the literature concerning ASW may be due to differences in the morphologies obtained by a variety of deposition methods They reported that the angular 23 distribution of incident H20 flux used to grow the ASW films in a vacuum is a critical factor influencing the ASW morphology at low temperatures They demonstrated that by systematically varying the incident angle of the H20 flux during deposition the controlled growth of nonporous to highly porous ASW could be achieved In astrophysical environments the morphology of ASW will depend on the two possible mechanisms of formation a direct deposition and b omni directi
46. agawa Y Inoue A Yabushita M Kawasaki Langmuir 16 9533 2000 BB Rowland and J P Devlin J Chem Phys 94 812 1991 4V Buch and J P Devlin J Chem Phys 94 4091 1991 K P Stevenson G A Kimmel Z Dohnalek R S Smith and B D Kay Science 283 1505 1999 SL Schriver Mazzuoli A Schriver and A Hallou J Mol Struct 554 289 2000 17S Mitlin K T Leung Surf Sci 505 L227 2002 BGA Kimmel K P Stevenson Z Dohnalek R S Smith and B D Kay J Chem Phys 114 5284 2001 E Mayer and R Pletzer J Chem Phys 80 2939 1984 2M S Westley G A Baratta and R A Baragiola J Chem Phys 108 3321 1998 IR Smoluchowski Science 201 809 1978 2V Buch and J P Devlin J Phys Chem 99 16534 1995 40 BM A Zondlo T B Onasch M S Warshawsky and M A Tolbert J Phys Chem B 101 10887 1997 ca Raut M Fama B D Teolis and R A Baragiola J Chem Phys 127 204713 2007 25A Givan A Loewenschuss and C J Nielsen J Phys Chem B 101 8696 1997 26A Bar Nun G Herman and D Laufer Icarus 63 317 1985 21A Bar Nun J Dror E Kochavi and D Laufer Phys Rev B 35 2427 1987 8E Mayer and R Pletzer Nature 319 298 1986 M Eldrup A Vehanen P J Schulz K G Lynn Phys Rev B 32 7048 1985 30N Horimoto H S Hiroyuki and M Kawa J Chem Phys 116 4375 2002 31S Andersson A Al Halabi G J Kroes a
47. age These computations investigate the dependence of the resolution of the image on the ion optics voltage Additionally a software program written using Microsoft Visual Basic 6 0 was employed to simulate the experimental pattern observed on the position sensitive detector from the photoproducts desorbed from the ice matrix This simulation will aid in predicting the velocity profile of the photoproducts and thereby will be valuable in proposing mechanisms for reactions in water ice and its constituents The synchronization of the instrumental components is a vital aspect of the experiment Therefore virtual instruments have been created using LabVIEW 8 6 to control the main elements of the experiment such as the Molecular Beam Valve triggering of photolysis 131 and probe laser pulses and high voltages power supplies Additionally software components were developed to record the TOF spectra of the ionized photoproducts to record the REMPI spectra and to acquire the image of the ion cloud Finally the preliminary work for the development of the VMI apparatus conducted through the photolysis of methanol ice at 157 nm photolysis using a different apparatus at the Kyoto University have been described This experiment provides an appreciation of the experimental parameters and exemplifies the capabilities of the state selective detection of photoproducts following the photolysis of the ice matrix Following this preliminary study it will
48. amount of energy as a consequence of undergoing collisions within the porous ASM structure before they are detected at the surface A fraction of the nascent CH is produced directly at the ASM surface and desorbs without interacting with neighboring molecules while sustaining a high translational temperature of 3000 K The formation of these two components of nascent CH is given in reactions 3 20 and 3 21 CH OH hv CH Trans 3000 K OH 3 20 CH 3000 K CH 90 K 3 21 119 In addition to this CH3 photoproducts that partake in secondary photoprocesses in the ASM bulk may remain undetected Jodkowski et al have reported the activation energies Ea available for hydrogen abstraction reactions by CH photoproducts in the gas phase reactions 3 22 3 23 AH E kJ mol CH i CH OH ads gt CH4 i CH2OH i 4 2 58 6 3 22 CH3 i CHOH ads gt CH4 i CH30 i 33 5 56 9 3 23 Hence the high temperature component of nascent CH with Eo 49 9 kJ mol may participate in this secondary photoprocess However a major fraction of nascent CH3 molecules is accommodated to the surface temperature and their average energy 2 7 kJ mol is insufficient to overcome the activation barrier of hydrogen abstraction Consequently the thermally equilibrated CH will be detected as primary photodissociation product At high photon fluxes CH3 may be further removed through the recombina
49. approach in VIs is the utilization of graphical programming code which makes use of graphs charts gauges and meters that are created to emulate an instrument With this programming approach LabVIEW has introduced a user friendly visual programming technique that allows the controlling of experimental conditions remote operation of instruments and analysis of data In comparison to traditional instruments the VIs created in LabVIEW provide more user defined and more application oriented solutions This chapter illustrates the programming of the instrumental components of the experiment using LabVIEW 8 6 The graphical development environment in LabVIEW has two main components which are the front panel and the block diagram The front panel is the User Interface UI which is the visual representation of the application It is typically characterized by the knobs buttons and controls which emulate the traditional instrument The inputs to the application are provided and the outputs are displayed using the front panel The 72 block diagram contains the graphical source code and reflects a flow chart rather than the usual lines of code seen in computer programs LabVIEW supports modularity and breaks down tasks to manageable sub tasks such as nodes and sub VIs Nodes in LabVIEW represent functions operators and subroutines in standard programming languages Structures are type of nodes that are commonly used in LabVIEW to exec
50. as a 2D image on to the detector positioned at the end of the drift tube The raw images may then be used to calculate the velocity and the kinetic energy distributions Z 44 The photofragments produced from photodissociation or photoionization depart with a fixed amount of kinetic energy KE as given in equations 2 1 and 2 2 respectively AB hv gt A B KE 2 1 AB hv gt AB e KE 2 2 The kinetic energy is portioned among photofragments according to the conservation of momentum as given in equations 2 3 and 2 4 AB Ke Ms xxe 2 3 M M KE l x KE 2 4 AB According to the mass portioning factor the lighter particles carry the larger fraction of kinetic energy Hence in a photoionization reaction the electrons carry most of the kinetic energy and in the photodissociation of a homonuclear diatomic such as Hp the total kinetic energy is shared equally between the two H fragments produced According to conservation of momentum each photodissociation reaction produces two partner fragments with equal momentum flying in the opposite direction of the center of mass frame These fragments are then able to create scattering patterns with a spherical distribution known as Newton spheres For a diatomic molecule with the transition dipole moment that is aligned parallel to the polarization of the photolysis laser beam the spatial distribution of photofragments assumes a cos distribution
51. bali Select Peak position Cursor Name TOF hitensity ix d x 4 Waveform 7 01784 1 Asie HE E 1 33333 1 6 000E 1 4 000E 1 2 000E 1 0 000E 0 2 000E 1 4 000E 1 6 000E 1 7 D 000101 FIG 28 Front panel of the Stepper Motor Scope REMPI VI 93 4 4 3 TOF and REMPI spectra panels When the voltage applied to the MCP exceeds 1500 V the MCP turns into the gain mode and can be used to gate ions of interest The TOF profiles are used to record ions as a function of wavelength of the probe laser The TOF spectrum is then exported from the oscilloscope to the computer using the DPO3034 sub VI and is displayed in the Waveform graph on the front panel in the Stepper Motor Scope REMPI VI The waveform graph has a pair of horizontal cursors and four vertical cursor units Each vertical cursor unit is comprised of a start and end cursor In order to form the REMPI spectrum the user must select the desired peak for analysis by positioning the cursors The execution of this program can be initialized by the REMPI Collect button The REMPI spectrum is produced by calculating the area of the TOF peaks of interest as a function of wavelength The REMPI spectrum will then be displayed in the REMPI Waveform panel and the spectrum can be saved to a user specified location with the REMPI save button 4 5 Imaging VI The SensiCam system consists of two major component
52. ch gives rise to the high frequency form The higher and lower frequency features of each doublet have been assigned to 2 coordinated and 3 coordinated water molecules respectively 15 monvacwtorco noe FES r 2800 2780 2760 2740 2720 2700 2680 Wavenumnbers FIG 3 Infrared spectra of thin films of amorphous ice in the OD stretching mode region a Pure D2O at 15 K b unexchanged 50 D20 50 H20 at 15 K c and d sample of part b annealed 10 min at 60 and 120 K Adapted from Rowland et al 16 Buch and Devlin reported that upon annealing the amorphous ice film to 60 K the 2 coordinate high frequency component of the doublet was entirely diminished However the low frequency component continued to remain in place When amorphous ice is further annealed to 120 K all evidence of the dangling OH disappears entirely This disappearance is a consequence of the surface restructuring that take place alongside the annealing process which leads to a reduction in the amount of internal surface in the solid ice Similar results are exhibited by the spectra for the OD stretching mode and its IR absorption features are presented in Figure 3 as a function of both temperature and fraction of D20 2 2 2 1 Temperature and pressure dependence of the dangling OH bond The temperature and pressure dependence of the IR absorption features are imperative to understand the formation of microporous ice at low temperatures
53. cifying the threshold trigger level slope and termination impedance of the external trigger input The burst mode can be controlled by specifying the trigger rate the burst period and number of pulses per burst 83 4 3 Data Acquisition with Lab VIEW The primary Data Acquisition DAQ processes used in this section generate signals analysis and presentations on the computer This process is implemented by utilizing the main components of DAQ which are data signals DAQ hardware and driver applications Using these components it is possible to use counters and timers to generate pulses and pulse trains with LabVIEW Virtual channels for data acquisition triggering and synchronization can be created with LabVIEW palettes and Sub VIs 4 3 1 Triggering The triggering of events in data acquisition can occur in three possible ways 1 External triggering 2 Internal triggering 3 Software generated triggering External triggering occurs when an interface uses digital pulses from specialized hardware or equipment to synchronize events Internal triggering is common in DAQ devices and occurs when an inbuilt comparator detects the signal crossing of a specified signal level and in turn triggers an event In software generated triggering a software program is responsible for the occurrence of event such as data acquisition In this section a LabVIEW program is used for data acquisition through NI PCI 6602E card which is plugged
54. d to be less than 2 nm The conventional four tetrahedral hydrogen bonds in water molecules are indicative of the condensed phase of HO However Rowland et al observed that water molecules on the pore surface cannot form these 4 coordinated tetrahedral hydrogen bonds They reported that as a consequence dangling O H groups that are weakly hydrogen bonded are formed on the pore surface Vibrational spectroscopic studies of vapor deposited ice films by Buch and Devlin showed evidence of dangling O H bonds for ice films deposited at low temperatures The existence of these dangling O H bonds designates vacancies in the ice network that are a distinctive feature of microporous ice films The vacancies denote the tendency to form hydrogen bonds with incident molecules as a consequence of the highly polarized surface Consequently an incident molecule with a hydrogen bond receptor or donor group such as HCl will have a higher probability of sticking to the surface and leading to hydrogen bond formation 14 Two kinds of dangling bonds are signified by a doublet at 3720 and 3696 cm in low density amorphous ice deposited below 15 K and at 2748 and 2727 cm for D20 as shown in Figure 3 a Dangling O H belonging to 3 coordinated molecules with two hydrogen bonds via O and one via H which gives rise to the low frequency form b Dangling O H belonging to 2 coordinated molecules with one hydrogen bond via O and one via H whi
55. deposited on solid Ar has been observed by Gerakines et al The photochemistry of the water molecule is initiated by reaction 1 8 H 0 hv H OH 1 8 Subsequent steps lead to the production of H202 and HO as given in reactions 1 9 and 1 10 OH OH gt H0 1 9 OH H O H2O HO 1 10 H2072 has been observed as a minor component of the water ice on the surfaces of Europa a satellite of Jupiter and of Enceladus one of Saturn s icy moons The UV photolysis of nitrate on snow grains produces H202 which is reported as a precursor for the production of OH in polar air through secondary photolysis In a study on the formation of H203 at the ice surface following the photodissociation of ASW at 90 K Yabushita et al proposed a mechanism for this process via reactions 1 11 1 13 H20 hv H OH 1 11 37 OH OH H202 1 12 H202 ho 300 350 nm 20H 1 13 Molecular dynamics calculations performed by Andersson et al to simulate the photodissociation of water ice at 10 K showed that OH radicals move at most 5 A through bulk ice However OH radicals released from photodissociation at the surface were reported to be highly mobile with the ability to travel 80 A over the surface Since there is appreciable mobility at 90 K the distance between many OH radicals will be comparable with their bimolecular reaction radius so that OH recombination reactions can 40 produce HO on the ice
56. e the unit trips and the high voltage is disabled If this occurs a VTRP voltage trip message appears in the panel The Source Setting sliding switch allows the user to specify the source before setting up its limit The default has been set to voltage and the switch manipulates the voltage setting provided that the front panel switch is turned on the instrument If the switch is in the OFF position while trying to set the high voltage the high voltage is left off and an execution error is reported The Enable Voltage Inversion control changes the voltage from positive to negative All of the above mentioned inputs are directed to the VISA Write Function in LabVIEW and the data is then written into the device 4 2 DG 535 Digital Delay and Pulse Generator The DG 535 can produce four digitally controlled time intervals These time intervals can be controlled by using a LabVIEW program through the GPIB connector The IEEE 488 2 connector allows the computer to control the DG 535 The DG 535 consists of five delay out BNCs named To A B C and D To sets the start time for the timing interval when the DG 535 receives a pulse from the master trigger The LabVIEW program allows the manipulation of the delays in the channels A B C and D The delay outputs A B C and D can then be set with respect to To or each other from 0 to 1000 s with increments of 5 ps For example the delay menu can be set as 81 A Tot 4 ns B
57. e angles or by background deposition N2 adsorption is highly dependent on growth temperature They observed an 26 extraordinarily large surface area of 2700 m g at 22 K and 640 m g at 77 K for ASW films grown from background deposition and reported a linear increase of porosity with time With these observations they proposed that large surface areas observed at low temperatures and oblique angles of the incident were a result of the connection of the internal surface being connected to the external surface of the film through a network of pores The porous character of low density amorphous ice is caused by thermo kinetic deposition The incoming water molecules that adsorb at the surface may lose most of their energy well before finding the most stable site 27 N adsorption monolayers N3 on 60 80 100 Deposition temperature K FIG 6 Amount of N adsorbed versus growth temperature for 50 bilayer ASW films The films were deposited by collimated beams at the angles indicated also shown are data for ASW films grown using background H O dosing The dotted lines through the data are to guide the eye For the thin films and low deposition rates used in these experiments the ASW film temperature and the Pt 111 temperature are essentially identical A T lt lt 0 1 K Reproduced from Stevenson et al 0 20 Ne 0 si e A aod A 15 10 0 45 60 Background 120 140 2
58. eact with the solid methanol matrix to produce secondary photoproducts 102 CH OH irradiation p F Band Area em G 10 3 4 10 10 100 1000 Irradiation time s FIG 32 Evolution of features in CH3OH irradiation MF methyl formate Reproduced from Gerakines et al Hama et al have reported the mechanisms for hydrogen atom abstraction recombination and hydrogen molecule elimination from solid methanol photodissociation at 90 K reactions 3 6 3 9 using REMPI technique CH30H hv H CO H gt 3 6 CH OH hv CH OH H 3 7 CHOH hv CH O H 3 8 103 CHOH H CH OH H gt HCO hvo CO H H2CO HCOH CH30H CO H2CO hvo CO H H2CO H CO H3 COHCO CH 0H hvu CH 0O D However reaction 3 14 is not observed in the gas phase Gerakines et al reported that further investigation was required to determine if methane was a primary photo product of photodissociated solid methanol This chapter reports the mechanisms for production and the possible reactions of CH3 and OH from the 157 nm photodissociation of Amorphous Solid Methanol ASM at 90 K using REMPI to determine their translational and internal energy distributions Possible second order reactions on ASM are also discussed An attempt was also made to detect O D and OCP 104 3 9 3 10 3 11 3 12 3 13 3 14 5 2 Experimental The photodissociation of so
59. ectral parameters available in current literature The two photon absorption cross section reported by Greenslade et al allows the estimation of the OH v 1 OH v 0 population ratio 5 4 Results 5 4 1 Kinetic energy and rotational energy distribution of CH3 Figure 35 a shows the REMPI rotational spectrum of CH v 0 products following the 157 nm photolysis of freshly deposited ASM recorded at a fixed delay of t 6 0 us The spectrum is assigned to the to 4p 7A v 0 xX A v 0 transition The spectral simulation shown in figure 35 b determines the rotational temperature Trot to be 15050 K lt E gt 1 20 4 kJ mol Figure 36 shows the congested Q branch in the TOF spectrum of CH monitored at a REMPI wavelength of 286 0 nm This 110 spectrum was reproduced by the sum of two MB distributions with Trans 3000 1000 K lt Etrans gt 49 9 16 6 kJ mol and Trans 90 20 K lt Eprans gt 1 5 0 3 kJ mol as shown in Table 3 The signal intensity of the Tian 3000 K component was weak Hence the rotationally resolved REMPI spectrum has been difficult to characterize in terms of Trot a i i h A nr Noman eat WS Neen Pe thal LU NARA EELA ELE EA D f i TS A ys i a E d 285 286 287 Wavelength nm FIG 35 a 2 1 REMPI excitation spectrum of CH radicals from the 157 nm photolysis of a fresh ASM sample at 90 K recorded at t 6 0 us b Simulated REMPI
60. ed to detect state selectively nascent OH and CH photofragments following photolysis at 157 nm Information on the velocity distributions was obtained from time of flight measurements ii At Queen s University Velocity Map Imaging combined with resonance enhance multiphoton ionization REMPI will be used for quantum state selective detection of the nascent photoproducts and their velocity distribution To help automate the experiments virtual instruments have been created for the hardware components of the experiment using LabVIEW 8 6 The ion optics of the velocity map imaging spectrometer under construction at Queen s have been characterized using the SIMION 7 0 software package and the anticipated experimental image of nascent photoproducts has been simulated by a Monte Carlo type algorithm iii Acknowledgements Many people have contributed to the completion of this thesis I would like to express my deep felt thanks to Professor Hans Peter Loock for his guidance and encouragement and Dr Jack Barnes for his assistance through this research project I am grateful to my co workers in the Velocity Map Imaging group Dr Wei Guo for the help with LabVIEW programming Jeff Crouse and Stephen Walker for the useful discussions I would like to thank my peers in the Loock Lab Jessica Litman John Saunders Klaus Bescherer Hanna Omrani and Helen Wachter for their friendship and support I am grateful to Professor Masahiro Kawasa
61. elerate the desorbed and resonantly ionized photofragments into a field free drift region Upon ionization charged particles form one or several nested Newton spheres which collapse into two dimensions along the TOF axis for each particular mass In conventional TOF mass spectrometers ions of the same kinetic energy but with velocities in different directions arrive at the detector at different times When the velocity map focusing conditions are met ions of the same mass that were formed at a given time with the same velocity converge on the same point on the detector regardless of their position of origin Since the drift region of the TOF chamber is of fixed length the convergence of the beam depends on the ratio of the voltages applied on the extractor and repeller Vg Vr The SIMION 7 0 software package allows the calculation of ion trajectories that fly through 3D electrostatic fields that assume cylindrical symmetry The SIMION virtual optics bench was used to simulate the three open hole electrodes each 11 4 cm in diameter and a TOF chamber that is 74 cm long The electrode geometries and potentials at each point in geometry are stored in a potential array and the voltages applied to the 55 electrodes were then manipulated as required The modeled spectrometer is given in Figure 12 FIG 12 3D TOF simulated by SIMION 7 0 with repeller extractor ground electrodes and detector Five ions of equal mass and wit
62. eller extractor and ground electrodes and an image detector as represented in Figure 9 The images produced using the ion optics assembly in ion imaging and velocity map imaging are compared in Figure 10 Furthermore VMI improves ion imaging by magnifying the radii of the ion images Eppink and Parker defined this radius R as R Nvt 2 7 where v is the expansion speed t is the time of flight and N is a magnification factor that depends on the experimental setup and electric fields 47 Ion optics Drift Region Assembly Molecular Beam Detector CCD Laser Beam FIG 9 Schematic representation of the instrument set up in velocity map imaging Ion imaging Velocity map imaging FIG 10 Comparison between images of O ions from the photolysis of molecular oxygen at 225 nm Reproduced from Eppink and Parker 48 Chang et al made a significant contribution to the technique by improving the resolution of the images further They analyzed each spot detected by the CCD camera where the burst of electrons created a blur in the image as a result of several pixels being excited and eliminated this problem by introducing real time event counting The highest intensity pixel was selected and stored for each ion that struck the phosphor screen so that the pixel values correspond directly to the number of ions detected at each pixel position The raw images from the CCD camera are in fact 3D projections
63. ermo Scientific Nicolet 6700 The FT IR spectra allow the characterization of the ice matrix Since FT IR is used to study the water ice surfaces the beam path has to be free of water vapor in the air This has been achieved by using an acrylic housing for the beam path that is filled by gas from a purge gas generator The produced photoions are accelerated in a VMI field and drift through a flight tube 74 cm onto the charged particle detection system The charged particle detection system consists of a MCP Chevron Burle Electro Optics Inc with a diameter of 75 mm and a phosphor screen Voltages are applied to the MCP to gate selected ions using high voltage power supply Keithley Instruments 245 and 246 The images of the ions that strike the phosphor screen are detected using a CCD camera Optikon 12 bit 1280 x 1024 resolution and the images are read out and are stored in the lab computer The schematic of the experimental set up is given in Figure 11 52 rate N pump 1 Cold Tip ueos AN s A Y 4 Turbo molecular Rs P a CCD pump 2 MC FIG 11 Top view of the experimental set up 53 The studies on physical properties of cryogenic ices especially related to ice surfaces the phase transitions of ice and the studies of matrix effects on the vibrational spectra of 21 25 However the molecules have received significant attention in the recent years properties of
64. es Figure 1 Structural dependence of ice on temperature 9 Figure 2 Temperature dependence of the IR absorption spectra of H O adsorbed on an Au 111 surface at 98 12 Figure 3 Infrared spectra of thin films of amorphous ice in the OD stretching mode region 16 Figure 4 Infrared absorbance spectra of unannealed 10 K binary mixtures of H2O mole fraction 33 and other components 66 20 Figure 5 Amount of N adsorbed by ASW films versus film thickness 26 Figure 6 Amount of N adsorbed versus growth temperature for 50 bilayer ASW films 28 Figure 7 Structure of a cosmic dust particle 33 Figure 8 Nested Newton spheres photofragments A and B where m gt mg 46 Figure 9 Schematic representation of the instrument set up in velocity map imaging 48 Figure 10 Comparison between images of O ions from the photolysis of molecular oxygen at Figure 12 3D TOF simulated by SIMION 7 0 with repeller extractor ground electrodes and detector 56 Figure 13 The xy plane view of the simulated TOF
65. f micropores in the condensed ice network 29 Microporosity is related to the retention of gas in amorphous ice Studies of gas adsorption on vapor deposited amorphous ice have revealed that the amount of gas uptake is consistent with microporosity and large surface areas of up to hundreds of m g However parameters such as pore shape structure and adsorption energies remain unknown In a study on the characteristics of pores using gas adsorption isotherms Raut et al reported the existence of dual pore structure in the form of mesopores and micropores They observed that the CH4 adsorption isotherms for ice films formed by background deposition of water vapor were different from those for collimated beams Films formed by background deposition demonstrated a step in the isotherms and less adsorption at low pressures This observation led them to propose that the micropores formed by these two methods were dissimilar In addition they reported that films deposited at 77 incidence to the surface from a collimated beam developed both micro and mesopores Upon annealing to 140 K where the ice crystallizes the micropores were destroyed while the mesopores were sustained 2 3 3 Trapping of gas The studies on the ability of water ice to trap and release gas have important implications linked to the outgassing process of comets 7 These studies help characterize the pores connected to the outside of the ice but not the e
66. features in H2O change when such a molecule donates electrons to the H2O network A new hydrogen bond O H B is observed in the H2O network In the case of a weak base the HOH B bond is weaker than the existing HOH OH bond Such molecules will be distinguished by an OH stretching frequency found between the main OH stretching band of fully hydrogen bonded H2O molecules and the 3700 cm band of 9 17 the free OH groups Hence the spectral signature of a weak base is comparable to a non bonding incident molecule With increasing base strength the OH stretching frequency of these molecules will decrease and the librational and bending frequency will increase The intensity of the OH stretching frequency also increases due to two factors Firstly the strong base pulls more H2O molecules out of the H2O network than weak bases do Secondly when the O HB bond is strong the polarization induced by the base causes an increase in the intensity of OH stretching vibration in every H2O molecule that forms a hydrogen bond 21 with the base The spectra of mixtures of HO and strong bases such as CH3 NH and NH reflect this behavior Consider an incident molecule HA with the ability to donate hydrogen atoms and accept electrons to create hydrogen bonds Such a molecule is considered to have characteristics of an acid according to the Bronsted and Lewis concepts of acids and bases e g HCI A is a conj
67. ge i j Next j Write 4 image i j 1 Next i Close 4 End Sub Private Sub Command 2_Click End End Sub Private Sub Command3_Click Call modBMP pCreateBMPImage BMP_ Palette _bw bny Monte Carlo Image raw Monte Carlo Image bmp Load frmBMP Info frmBMP_Info Show End Sub 137
68. hout initial kinetic energy were generated in the region between the repeller and the extractor by first defining the position of the first ion and then incrementing the position of the others by 1 mm in the y direction i e perpendicular to the TOF axis The simulation was repeated for different voltage settings until the trajectories with the same initial velocity but different initial positions were converged to a single point on the detector This is the velocity mapping condition The optimal focusing voltage for Ve Vr was found to be 0 55075 After setting the extractor voltage division in focus the setup is ready to collect ion and electron images that do have 56 kinetic energy release If the laser position and Vp Vpg ratio are kept the same the only variable is the repeller voltage Vr to obtain images in the size that is desired Now images of any ion mass can be obtained without having to change the Vp Vp ratio This simulation is illustrated in Figure 13 R E G Detector x FIG 13 The xy plane view of the simulated TOF When V Vr meets the focusing condition all ionized species with the same mass and the same velocity reach the MCP detector regardless of the location they were formed In the next simulation given in Figure 14 the magnification factor for the image was calculated The magnification factor is the factor by which the total image size is larger than expected based
69. indicative of the highly inter connected pore structure that allows N2 to penetrate the ASW film Stevenson et al reported that the N2 adsorption on ASW ice films grown at normal incidence was much smaller and almost independent of thickness Moreover they found that the ASW behaved as a denser film without a network of pores and that adsorption mainly took place through the external surface The results were in agreement with the ballistic deposition model which considers the effects of coarseness in the surface caused by random vapor deposition 25 O Q Qm 35 80 O 45 P 4 60 SSSAAA Background 9 O N adsorption monolayers N3 0 50 100 150 200 250 Film thickness bilayers H O FIG 5 Amount of N adsorbed by ASW films versus film thickness The films were deposited by collimated beams at 22 K at the angles indicated also shown are data for ASW films grown using background H2O dosing Fitted lines show a linear increase in N uptake with increasing film thickness Reproduced from Stevenson et al Stevenson et al also measured the Nz adsorption by 50 bilayer ASW ice films as a function of the sample temperature as given by Figure 6 They found that Nz adsorption decreased with rising surface temperature For ASW ice films grown at normal incidence this decrease was small since these are nonporous films independent of growth temperature In contrast for ASW ice films grown at obliqu
70. ion imaging 1987 when they combined use of position sensitive ion detection with Charge Coupled Device CCD cameras to provide a more sensitive technique to investigate the photodissociation process This technique was achieved by directing a skimmed CH3I molecular beam through a hole in the repeller plate of the ion optics assembly The molecular beam was then intersected by photolysis and probe laser beams at position between the repeller and the grounded grid electrodes The photolysis laser ruptured the C I bond at 266 nm and produced CH radicals and I atoms while the probe laser ionized the CH v 0 photofragment The potential between the electrodes was controlled so that the resulting CH ion cloud is accelerated through a Wiley McLaren Time of Flight Mass Spectrometer TOF MS on to the detector The electric field also compresses the CH3 ion cloud along the time of flight axis and all ions arrive at the position sensitive detector concurrently This phenomenon known as pancaking is crucial to minimize the blurring of the image The detector is composed of a position sensitive Microchannel Plate MCP and a Phosphor Screen PS The three dimensional 3D ion cloud is projected as a two dimensional 2D image on to the detector positioned at the end of the drift tube The pattern of the ions striking the detector can then be captured by a CCD camera and processed by using imaging software programs The 3D ion cloud is projected
71. ion occurs from CH30 and HCO CH 0H hv CH 0 H 3 1 CH 0H hv H CO 2H 3 2 CHOH hv gt HCO H 3 3 CHOH hv gt CO 2H gt 3 4 CH 0H hv gt CH OH 3 5 101 Satyapal et al investigated the possible bond cleavage in gas phase methanol at 157 nm and reported that the O H cleavage channel through reaction 3 1 was dominant with a quantum yield of 0 86 0 1 The remainder was thought to be through the C O cleavage channel via reaction 3 5 However this contribution had not been quantified The photodissociation of solid methanol is more complex than in the gaseous phase since the photofragments are trapped in the site of production Hence the photochemistry of solid methanol has received much attention during the recent years Gerakines et al have studied the UV photochemistry of pure methanol ice samples at 10 K through infrared absorption spectroscopy They reported the formation of carbon monoxide CO methyl formate Hx COHCO carbon dioxide CO2 methane CH4 formyl radical HCO formaldehyde H2CO and hydrogen H2 as given in Figure 32 which was indicative of a complex photochemistry for methanol However they could not detect the primary CH and OH fragments The possible reaction pathways for the products obtained from photodissociated solid methanol are given in reactions 3 6 3 14 The primary photo products from the photodissociation of solid methanol may further r
72. ki and his research group for hosting my visit in Kyoto University Japan The Queen s University Writing Centre has been a wonderful source for feedback on my writing and I am also thankful to Claire Hooker for proof reading this thesis I greatly appreciate the assistance of Dr Arunima Khanna and Barbara Fretz of Queen s University Learning Strategies My special thanks go to Barbara Schlafer Gamila Abdulla Karen Knight and Lisa Webb of the Ban Righ Center for the inspiration they provide and the numerous ways they have assisted me My heart felt gratitude goes to Eily Strotmann for adopting me as her granddaughter helping me every step of the way and for making Canada my home away from home I am grateful to Nancy Binks and Yolande Webb for helping me in my hour of need iv Finally I would like to thank my family I have benefitted greatly from the many sacrifices my parents Swarnamalie and Sarath Wickramasinghe have made I could not appreciate them enough for all the love they have given me I am thankful to my sister Sathika Wickramasinghe my twin soul my grandmother for her kindness Chatura Hewavitharana for being my strength my extended family for their support through the years and to Professor Ruchira Cumaranatunga for being a lifelong inspiration Thank you To Amma and Thaththa vi Table of Contents VOIN ELE A AT A E E E eae ogee eed ii Acknowledgements isc ccelsiedseccetags cess eE a E E E
73. lid methanol was performed in a high vacuum chamber equipped with two turbo molecular pumps operating in tandem Shimadzu 800 and 50 Ls 11 13 a pulsed molecular beam an excimer laser and two dye lasers The base pressure of the vacuum chamber was maintained at 5x10 Torr using the two turbo molecular pumps An optically flat sapphire substrate sputter coated with a thin polycrystalline film of Au 111 was mounted in the center of the chamber by a liquid nitrogen cooled manipulator The substrate temperature was controlled by heating a tantalum filament attached to the substrate ASM films were prepared by backfilling the deposition of methanol vapor on to the gold substrate at 90 K for 60 min with an exposure of 1500 L 1 L 1x10 Torr UV photoirradiation and multiphoton ionization detection of CH and OH were performed at a substrate temperature of 90 K A 157 nm laser Lambda Physik OPTexPro with a pulse duration of 10 ns was incident on the ASM surface at an angle of 80 to the surface at a fluence F lt 0 1 mJ cm pulse CH products were ionized at a distance of 2 mm from the substrate surface by 2 1 REMPI through the 4p A v O e X A v 0 transition at 285 287 nm and collected with a small mass spectrometer aligned perpendicular to the ice surface 4 105 OH products were ionized 4 mm above the substrate surface by 2 1 REMPI via the Ds v 0 e X M v 0 transition at 243 5 244 5
74. locity of the fragments If the normalization coefficient c is chosen so that 8 1 the remaining coefficients 2 are known as the spatial anisotropy parameters The onion peeling program transforms the image from Cartesian coordinates to polar 28 30 coordinates and by fitting the angular distribution to the above function and the anisotropy parameter 8 is determined for each radius The values for the spatial anisotropy parameters that were used to construct the simulation and those recovered from the onion peeling program and the Abel transform were then 65 compared A discrepancy between the simulation input and the output were observed because the velocity distributions are set very narrow during the inversion process and that throws off the program Therefore the simulation was repeated with velocity broadened distributions and the spatial anisotropy parameters were recovered The results are given in Table 2 while the inverted images are given in Figure 16 66 Simulated Image from the Visual Basic Program Inverted Image from the Onion Peeling Program Inverted Image from the Abel transform FIG 16 Typical image simulated by the Visual Basic program top using broadened velocity distributions the Inverted image from the Onion Peeling Program middle and the Inverted image from the Abel Transform bottom 67 TABLE 2 Anisotropy parameters for simulated image and inverted image
75. m returns a session identifier that can be used to call any other operations in the power supply This process is shown in Figure 2 Delay ms SEE Emi Mo SET YF False String Append True Append True False String VISA Close VISA Write FIG 20 Communication with the device 79 The front panel in Figure 21 allows the user to specify the exact voltages required using the start and end controls The ability to change the voltage in small increments is important to prevent damage to the assembly and also to observe the changes in the image when the repeller and extractor voltages are adjusted The step and increase decrease controls allow this manipulation The output voltage is displayed in the VCurrent box The output voltage cannot be set higher than the voltage limit OwoFF High oltage Control T On Default configure Repeller Plate Se Delay ms F 500 nees Source Setting T Voltage aj mood a Increase F Voltage ISA resource name Bo KGP ote 0 Baal End H 100 current D Enable Yoltage Inversion T Enabled error out m faa oO C Decrease Increase Li i o ie i Slj 8 il i o o O Decrease Increase 1 00 m 5 o o _ Decrease gt Increase po FIG 21 Front panel of power supply 80 In addition if the output exceeds 10 of the full scal
76. mageSize VI determines the number of horizontal and vertical pixels in the current 96 image configuration The scaling of the 12 bit image to a 16 bit image and normalizing it has been implemented using a two dimensional array Part of the block diagram is shown in Figure 31 FIG 30 Imaging VI front panel 2 to display images 97 ndie Ey Name b gt Camera type CTT iii New delay and exposure formatter vi Delay and exposure values ms For HP Ei Get info on mode of operation Camera information camera SCGetImageStatus vi ree type operating temperatur f Select the Initialize the Call stop function Run the Camera f board to use camera to reset camera Program the Operation Code Ae program control Camera Operation Code FIG 31 Imaging VI block diagram 4 6 Conclusion The synchronization of the elements in the apparatus is important in the experimental procedure This has been achieved by the virtual instruments created using LabVIEW 8 6 The programs written allow the user to set the experimental parameters and collect data using the data acquisition system Since the programs support modularity it is also possible to add other components to the same programs in the future without disrupting the existing modules 98 References for Chapter 4 IS Sumathi and P Surekha LabVIEW based advanced instrumentation
77. n in Figure 28 The Remote Control button starts the execution of the DPO3034 panel The VISA resource name of the oscilloscope can be selected from the drop down menu By specifying the VISA resource name the user allows the computer to acquire data from the oscilloscope through the USB cable The digital control for channels allows the user to display the waveform in the desired channel The Maximum Time control specifies 92 the amount of time to wait for the acquisition of data The default is set to 10000 ms The Save button saves the spectrum into the path specified by the user 4 4 1 4 Implementation of the oscilloscope The DPO3034 panel has been programmed with the following sequence of operations Initializing the session with the instrument sequentially acquiring data waiting until the operation is complete acquiring the waveform and closing the session The Single Waveform VI initiates acquisition based on the configuration information waits for the acquisition to complete and returns the requested waveform The Close VI 3 10 runs an instrument error query before closing the session Stepper Motor Start nmm im qhs055 io oo Lette bredo a ISA Resource Name Scope Maximum Time 10000ms TTL Pulse width runistep 3 00012 3 USB0 0x0699 0x0413 E 10000 He ed rt aeltas Corrent Position fi channel 2 Steps Loop 200 Current step ErrMsg 8 fioo pi QK179 1001 ste
78. nclosed pores 30 Gases can be trapped by condensation on the ice surface at low temperatures below gas freezing temperature or by the formation of clathrate hydrates in which the gas molecules are trapped in cages formed in the water ice only in the presence of the gas The high porosity of the amorphous ice allows large volumes of gas to settle in the vast number of cracks and holes When the volume is filled and a monolayer of adsorbed gas is formed 26 the surplus gas freezes on the ice surface Bar Nun et al reported that CO CH4 N and Ar gases trapped in are released at four temperature ranges a 30 60 K some of the holes are reopened and the gas frozen on water ice evaporates while from 80 120 K the annealing locks gas inside a compact and impermeable matrix b 135 155 K the trapped gas is squeezed out during the transformation from amorphous into less porous cubic ice c 160 175 K deeply buried gas is released during the transformation of cubic ice into hexagonal ice d 165 190 K gas and water are released simultaneously during the evaporation of a clathrate hydrate With increasing growth temperature the gas absorption ability or porosity decreases greatly ASW formed at temperatures below 90 K is a highly adsorbent solid with a surface area of approximately 400 m g At temperatures above 90 K Stevenson et al observed that ASW films essentially had the same adsorption as the non po
79. nd E W Dishoeck J Chem Phys 124 064715 2006 32M Greenberg and J I Hage Astrophys J 361 260 1990 3E Herbst Chem Soc Rev 30 168 2001 sa a N Allamandola S A Sandford and G J Valero Icarus 76 225 1988 3P A Gerakines W A Schutte J M Greenberg and E F van Dishoeck Astron Astrophys 296 810 1995 3A Yabushita N Kawanaka D lida T Hama M Kawasaki N Watanabe M N R Ashfold and H P Loock J Chem Phys 129 044501 2008 37A Yabushita N Kawanaka D Iida T Hama M Kawasaki N Watanabe M N R Ashfold and H P Loock Astrophys J 682 L69 2008 38G Manico G Raguni V Pirronello J E Roser G Vidali Astrophys J 548 L253 2001 R W Carlson M S Anderson R E Johnson et al Science 283 2062 1999 4A Yabushita D lida T Hama and M Kawasaki J Chem Phys 129 014709 2008 D D Davis J B Nowak G Chen M Buhr R Arimoto A Hogan F Eisele L Maudlin D Tanner R Shetter B Lefer and P McMurry Geophys Res Lett 28 3625 2001 41 R Weller A Minikin G Konig Langlo O Schrems A E Jones E W Wolff and P S Anderson Geophys Res Lett 26 2853 1999 8A E Jones R Weller A Minikin E W Wolff W T Sturges H P McIntyre S R Leonard O Schrems and S J Bauguitte J Geophys Res Atmos 104 21355 1999 A E Jones E W Wolff J Geophys Res Atmos 108 4565 2003 SA Yabushita
80. nm and also the Ds v ile xiy 0 and 35 hv O e Xv 1 transitions at 237 5 237 7 nm The rotational structure of the of the measured spectra was simulated using the PGOPHER software package and thus the rotational temperatures of the products were determined Figure 33 illustrates the experimental set up REMPI laser Photolysis laser _ at Photolysis r REMPI laser _ lt gt 1 laser MQ Time FIG 33 Schematic illustration of the experiment Photofragments from the 157 nm photodissociation of ASM are detected by the REMPI technique The timing plot for measurements of a TOF spectrum is shown in the inset where the delay time between photolysis and probe laser pulses corresponds to the TOF r is the vertical distance between the disk substrate and the REMPI detection point The VUV photoirradiated area is a disk with a radius of 6 mm Reproducd from Hama et al 106 Detection of 0 D 0 P atoms by 2 1 REMPI via the o F lt D transition at 203 8 nm O P P transition at 225 6 226 4 nm was unsuccessful as no signal was observed Yabushita et al and Hama et al have previously detected O D OCP 10 atoms by the same REMPI setup from the 157 nm photolysis of amorphous solid water at 90 K The possibility of the measured photoproduct distributions being influenced by the photoproducts that are accumulated on the surface af
81. nncdsouate 30 2 4 Photolysis of A S W issctienctescs ds casurteoinadadavicinteiehonsadinoesa ibneteadtsg abe aaa a A 32 2 5 Conclusio a 28s Ck a8 5 TaN a E ERE otis Cae eV EE AN C3 Touts oh Tiny 39 References for Chapter 2ra o cence Sie du tian bias wndingead A adhe ROEN en 40 Chapter 3 Velocity Map Imaging and Simulations 00 0 cc ccecce cence ene e eee ene ene ensaaeens 43 3 0 Introduction 364 2 5 5b 652s BE Rede eagle Sec RNAS ee RR E ee ek 43 3 1 Experimental Set UP cursori enep EE KE E see id AEE AE EES 50 vii 3 2 SIMION Simulation Ses aena dadege view nes EE RAOS E OETA avs Seba neue interne mneees 55 3 3 Monte Carlo Image Creation ccc cece ence cence ee ene nce e ee nee eee ene ene enn ees 59 3 4 CONCIUSION eee E pes canes E ae KA A dew euaanek eek A OEA de habe ed nedveweeks 68 References for Chaptet 3 3 123 lt i2seax invaders whet Riad seeds R ENE E E AE Rian 70 Chapter 4 Programming of Instrumental Components in the Experiment 000000s 72 4 0 Programming with LabVIEW ccc ccc e cence eee e ene nent ene nent teen eee re 72 4 1 PS 350 series High Voltage Power Supply cceceeceeceeceneeee ene enee eee ensns 79 4 2 DG 535 Digital Delay and Pulse Generator 00 ccc ece eee ee eee teeta tenes en ens 81 4 3 Data Acquisition with LabVIEW ccccecee cence ence nee e nen nee e eee eens eee nes 84 4 3 1 PPI POSTING ia shade e A O a ad E R T E E E EE
82. nt of the species include positive ions e g H3 HCO H30 radicals e g C H through n 8 isomers of stable compounds e g HNC HCCNC HOC and unusual three membered rings e g C3H C3H2 Of course molecular hydrogen is found to be the dominant species The second most abundant species is CO The concentration of molecular hydrogen is reported to be 10 times higher than that of CO Chemical process on ice mantles can be categorized into two types 1 Energetic processes caused by radiation 2 Non energetic surface reactions in which abundant atoms such as hydrogen and oxygen play an important role Photons in the range from UV to Vacuum Ultraviolet VUV can induce chemical reactions on ice mantles Dust shielding a dense molecular cloud makes it difficult for UV and VUV photons to penetrate the cloud The internal UV flux in a dense cloud is approximately 10 photons cm s Although this flux gives one incident photon per month on a grain a significant chemical evolution takes place over the life time of a molecular cloud which is 10 10 years Hence certain processes induced by UV 31 absorption are important in evolving molecular clouds 34 Within the characteristic temperature range of 10 100 K in molecular clouds molecules are typically synthesised through barrier less ion molecule reactions in the gas phase and 4 31 34 d However therefore these gas phase
83. o VUV radiation the H O bond ruptures and this is reported as a photolytic source of hot hydrogen atoms at the surface of comets and dust grains in the interstellar medium Yabushita et al reported that hydrogen molecules on ASW are produced by two distinct mechanisms hydrogen abstraction HAB reaction 1 6 and hydrogen recombination HR reaction 1 7 HAB H HOH gt H OH 1 6 HR H H gt H 1 7 Since the optical penetration depth of a water ice film at 157 nm is 100 nm Yabushita et 36 37 al proposed that H atoms produced far beneath the ice surface will collide with surface OH groups that are exposed through the porous surface of ASW to produce hydrogen molecules Andersson et al reported that unlike in a molecular cloud multiple photodissociative events can take place on the water ice surface within a narrow range of time and space when exposed to a high flux of UV photons Photofragments released from different sites 36 can react with one another to produce species such as OH HO Oz and H20 gt Some photofragments are observed to be mobile and may move distances of several angstroms before becoming trapped in sites and they can partake in additional reactions However the recombination process such as the reaction of H and OH to form H2O limits the possibility of the photofragments reacting further The presence of OH HO and H202 products in UV photolysis of ice
84. of Newton spheres on a 2D screen In order to extract the kinetic energy and angular distribution information from the images it is necessary to reconstruct the original 3D distributions Suits et al have developed a method by which only the centre slice of the ion sphere is recorded and thereby circumvents these mathematical reconstruction methods His DC slicing method is used to stretch the ion sphere rather than pancaking it and the detector is only turned on when the centre fraction of the sphere arrives This necessitates the use of additional electrodes and electric fields but has been widely adopted especially for heavier fragments In addition there are two fundamental approaches used in literature to reconstruct the 3D distribution from the projected full ion sphere a inversion methods and b forward convolution methods The inversion methods assume that if the 3D distribution has an axis of cylindrical symmetry and if a 2D central slice is taken through the 3D distribution the slice contains 49 L16 From this slice the all the information required to reconstruct the original distribution 3D projection is obtained by rotation around the cylindrical axis a process known as the Abel transformation The inverse Abel transform can be used to obtain the slice from the projection of the 3D distribution In an experimental point of view in a photodissociation process if the polarization of the photolysis laser i
85. of substrate temperature At 35 K the density of ice films was found to be 0 68 g cm This is considerably lower than 0 93 g cm for ice J This low density is due to the formation of microporous amorphous ice Furthermore the density increased rapidly from 0 68 0 78 g cm for temperatures between 30 60 K and from 0 80 0 93 g cm for temperatures between 80 K 120 K A density of 0 93 g cm was reported for ice films between temperatures 120 K 150 K The structure of unannealed amorphous ice is found to have greater dispersion of OOO angles and nearest neighbor O O separations and a larger mean separation than that of annealed amorphous and polycrystalline ice This gives rise to a more disordered structure in unannealed amorphous ice and results in a wider distribution of weaker hydrogen bond strengths Of the solid forms of H20 the unannealed amorphous ice is the most comparable structure to that of liquid water The transition from J h to Tal is accountable for the diffusion and recombination of radicals of interstellar ices processed by ultraviolet radiation at low temperatures 10 2 2 Spectroscopic studies of vapor deposited ice films 2 2 1 Pure H O The infrared spectra of all forms of solid water are characterized by four absorption bands A sharp band is observed for the dangling OH stretching mode from the ice surface at 3700 cm This intense band corresponds to the symmetric v and antisymme
86. olve from it Chapter 5 of this thesis discusses in the photolysis of methanol ice 2 5 Conclusion Chemical reactions on the surface of cosmic ice dust play an important role in chemical evolution Among the many kinds of molecules observed the abundances of some major species such as hydrogen molecules cannot be explained by gas phase synthesis Therefore surface reactions on cosmic dust are considered for the synthesis of such molecules Further research on the reactions that takes place on the water ice surface is desirable in order to understand the formation and evolution of icy grains in molecular clouds 39 References for Chapter 2 M A Tolbert A M Middlebrook J Geophys Res 95 22423 1990 P Jenniskens and D F Blake Astrophys J 473 1104 1996 R A Baragiola Planet Space Sci 51 953 2003 AN Watanabe and A Kouchi Prog Surf Sci 83 439 2008 gt A H Narten C G Venkatesh and S A Rice J Chem Phys 64 3 1976 C A Angell Annu Rev Phys Chem 55 559 2004 1S Mitlin and K T Leung J Phys Chem B 106 6234 2002 SP J enniskens and D F Blake Science 265 753 1994 A G G M Tielens W Hagen and J M Greenberg J Chem Phys 87 4220 1983 10B S Berland D E Brown M A Tolbert and S M George Geophys Res Lett 22 3493 1995 HW Hagen A G G M Tielens and J M Greenberg Chem Phys 56 367 1981 125 Sato D Yamaguchi K Nak
87. on factor for the anisotropy parameter In Visual Basic the Sgn function returns an integer that indicates the sign of the operation The Visual Basic code has been included in the appendix section The calculated co ordinates of each ion are stored in an array An image is calculated as counts of the number of events falling within each pixel The code given below illustrates how binning into an image with a resolution of res x res pixels can be used to increase the intensity of the pixels if two ions with the same co ordinates are generated in a binned image Rmax is the maximum radius of the image Yimage INt Y Rmax 1 res 2 Zimage Int Z Rmax 1 res 2 IntImage Y image Zimage i IntImage Yimage Zimage 1 64 To verify that the simulation code actually works the 3D simulated image was then inverted with the onion peeling program to obtain a 2D slice through the center of the image The 2D image provides information about velocity profiles and spatial anisotropy parameters and is expected to recover the input parameters from the simulation The program calculates angular distribution at a given radius The angular distributions resulting from an ionization process can be expressed as a sum over Legendre polynomial components 1 0 c gt B P cos8 2 16 Where N depends on the number of photons driving the process and is the angle between the axis of polarization of the photolysis light and the ve
88. onal deposition Direct deposition may occur in the rings of Saturn where it has been reported that water molecules sputtered from the outer rings accumulate on the inner rings it may also occur when a body such as a dust grain is passing through a molecular cloud with a large relative velocity Porous ASW may form by means of omni directional deposition of water molecules and may occur during the formation of comets In the light of these occurrences the morphology of thin ASW films has been investigated by two different methods a Using highly collimated effusive H20 beams i e direct deposition b Introducing water vapor through a leak valve 1 e background deposition Kimmel et al observed that beam deposited ASW films had an area of uniform thickness surrounded by an area of decreasing thickness contained within the diameter of the sample The films grown by ambient vapor were uniform across the sample 24 Stevenson et al compared the N adsorption of ice films varying in thickness grown using both of these methods between 22 145 K by dosing at different incident angles For large incidence angles the Nz adsorption increased almost linearly with thickness as given by Figure 4 The N2 adsorption of ASW films grown by the background deposition method was higher than the adsorption of ASW films grown by collimated H20 beams at high angles of incidence The linear increase of the Nz adsorption is
89. or CH30H 5 6 Conclusion The formation of photofragments CH v 0 and OH v 0 and 1 from the C O bond cleavage channel has been investigated through the 157 nm irradiation of amorphous solid methanol at 90K The low internal energies of the CH3 photofragments suggest that the majority of the CH photofragments are accommodated to the substrate temperature and diffuse to the vacuum interface through the ASM bulk phase However the detected OH photofragments originated entirely from the ASM surface and any OH formed in the bulk phase of amorphous solid methanol is trapped or reacts with molecules in the bulk The TOF spectra of CH photofragments from CH30H H20 ice sample suggest that the reactivity of solid water with CH products is small 124 As discussed in the literature review on chapter 2 similar reactions take place in amorphous solid water when dissociated species react to react further with water molecules Incident UV photons A lt 0 28 can cause the primary dissociation of 4041 The photodissociation of trapped OH can produce O water ice yielding H OH H H atoms may further react with the water ice molecules to produce H molecules through H abstraction or the H atoms may recombine to produce H reactions 3 6 and 3 7 This process is relevant to the formation of H by the photolysis of water ice on dust grains in the warmer regions of the interstellar medium HAB H HOH gt H OH 3 6
90. ows Dim NumberOfSamples i As Integer For 1 1 To NumberOfSamples Print Display Number i Next i The same loop written using the graphical code in LabVIEW will be as shown in Figure 17 Block Diagram Icons l Numeric Control Terminal 2 Wire Data Path 3 For Loop Front Panel Icons l Digital Control 2 Digital Indicator FIG 17 Implementation of a for loop in LabVIEW Block Diagram and Front Panel 74 The principle followed in the source code of the LabVIEW program is similar to the text based programming languages However LabVIEW is advantageous in terms of not having to declare or initialize variables as opposed to most programming languages An additional advantage is that LabVIEW simultaneously creates the front panel as the programming on the block diagram takes places LabVIEW also allows the utilization of built in tools such as Measurements Automation Explorer for programming the Data Acquisition Devices DAQ The standard I O language for instrumentation programming is Virtual Instrumentation Software Architecture VISA VISA is a capable of controlling General Purpose Interface Bus GPIB and serial instruments and call lower level drivers VISA resources operations and attributes commonly used in LabVIEW programming The VI communicates with a device by establishing a VISA session The basics of this process are used in LabVIEW programming as shown in Figure 18 A wie A
91. raction of the CH radicals being accommodated to the substrate 121 temperature 90 K This implies that the CH products desorbed without reaction with solid water 5 5 2 OH radical formation from the photolysis of fresh ASM The translational energies of OH v 0 and 1 from fresh ASM are characterized by the temperature Tirans 3000 500 K E ans 49 9 8 3 kJ mol The OH rotational temperatures 7 v 0 300 100 K and 7 v 1 200 50 K are not accommodated to the substrate temperature of 90 K This implies that the nascent OH photofragments originate from the ASM surface The lack of a thermally equilibrated OH component also suggests that OH formed in the bulk may be trapped or reacts with molecules in the bulk as given in reactions 3 20 and 3 26 Andersson et al reported that the desorption probability of OH was less than 3 per monolayer in the top 3 monolayers of water ice at 10K 34 35 CH30H Av 157 nm gt CH OH Ttrans 3000 K 3 20 OH 3000 K gt trapped or reacted with ASM 3 26 The OH radicals originating in the bulk may progress to a maximum distance of 5 Aat 10 K in bulk water ice and more than 60 A in the top 3 monolayers The presumable hydrogen abstraction reactions are given in reactions 3 27 and 3 28 The activation energies for the same reactions in the gas phase have been reported by Jodkowski et al 122 AH E kJ mol OH i CH3 OH ad
92. ratus at Kyoto University In the final chapter the state selective detection of OH and CH photoproducts from the 157 nm photolysis of methanol ice is discussed and their REMPI spectra analysed A plausible reaction mechanism and the energetics of the reaction are presented References for Chapter 1 G B Hansen T B MsCord J Geophys Res 109 E01012 2004 B A Smith L Soderbolem R Beebe J Joyce G Briggs A Bunker S A Collins C J Hansen T V Johnson J L Mitchell R J Terrile M Carr A F Cook II J Cuzzi J B Pollack G E Danielson A Ingersoll M E Davies G E Hunt H Masursky E Shoemaker D Morrison T Owen C Sagan J Ververka R Strom and V E Suomi Science 212 163 1981 F L Whipple Astrophys J 11 375 1950 4M A Tolbert A M Middlebrook J Geophys Res 95 22423 1990 T G Koch S F Banham J R Sodeau A B Horn M R S McCoustra and M A Chesters J Geophys Res 102 1513 1997 A B Horn T Koch M A Chesters M R S McCoustra and J R Sodeau J Phys Chem 98 946 1991 7S Andersson A Al Halabi G J Kroes and E W Dishoeck J Chem Phys 124 064715 2006 SL J Allamandola M P Bernstein S A Sanford and R L Walker Space Sci Rev 90 219 1999 N Watanabe and A Kouchi Prog Surf Sci 83 439 2008 E Herbst Chem Soc Rev 30 168 2001 Chapter 2 Literature Review 2 0 Introduction
93. river QK179 These devices are shown in Figure 25 FIG 25 Stepper motor controller and data acquisition device 90 4 4 1 1 Implementation of the stepper motor controller The address of the driver port and the direction of rotation in the stepper motor have been configured using VIs for the data acquisition device from Measurements Computing as given in Figure 11 These VIs allow the port to be configured as an output port and to set the direction bit The stepper motor program can be used from a stand alone VI to align the probe laser or from the QK179 panel on the Stepper Motor Scope REMPI VI to collect TOF spectra Select Port FIRSTPORTB output FIRSTPORTB DCfg Err Port Msg Direction FIG 26 Block diagram to change direction of rotation in the stepper motor 4 4 1 2 Front panel of the stepper motor controller The front panel of the sub VI is given in Figure 27 In it the user can specify the direction of rotation for the motor and the start wavelength The default settings for the pulse width have been set to 5 the delay has been set to 0 and the number of steps per 91 loop has been set to 200 Upon execution the current wavelength of the probe laser will be indicated in the panel FIG 27 Front Panel of the Stepper motor sub VI 4 4 2 Oscilloscope panel to store TOF spectra 4 4 1 3 Front panel of the oscilloscope The TOF spectrum is observed in the digital phosphor oscilloscope as show
94. rous 31 crystalline ice formed at 145 K Rowland et al reported that dangling O H bonds diminished by warming to temperatures near 60 K They concluded that micropores that continue to remain in ice above 60 K have no access to the surface In a study on the gas retention of ASW Baragiola reported that 1 7 nm cavities in ASW disappear or join together at 100 K but some cavities continue to remain even after warming beyond the crystallization temperature Upon annealing ASW becomes a compact solid with closed pores and this yields a reduction in gas adsorption Horimoto 1 investigated methane adsorption using infrared spectroscopy and proposed that gas eta adsorption was rendered by cavities larger than micropores which collapse upon annealing to 60 K They concluded that micropores are not affected until 80 K and collapse only at 120 K 2 4 Photolysis of ASW The effects of ultraviolet radiation on water ice are important in the chemistry of both atmospheric and interstellar ices Significant progress has been made in understanding the chemical and physical processes following the absorption of Ultraviolet UV photons by condensed phases of water Cosmic dust is formed from gases of refractory elements such as Mg Si O and C around 1000 K Dust diffuses to interstellar space and gradually forms a molecular cloud 32 Herbst has reported extensively on the formation of molecular clouds Watanabe
95. rphous Solid Methanol at 157 nm 0 0 eceeeeeee eee ee eee 100 5 0 Background cr a are 100 5 1 Introduction esseere ieir E E R EE E AE EEE EESE 100 5 2 Experimental zenist ireo E T E r E a OEE 105 5 3 Simulation of 2 1 REMPI spectra 0 cece ccc ec eee e eee neces eneaene ene eeenaenes 109 5 4 RESUIES sf ei ca cares wx osca nie EEE E eaten re cen ad Reba TENNA Me ESEESE TEORIKO 110 5 4 1 Kinetic energy and rotational energy distribution of CH3 ccceee eee eees 110 5 4 2 Kinetic energy and rotational distribution of the OH radical ee ee 113 5 4 3 Additional 157 nm photolysis experiments on ASM 0 cccceceeeee ene eneenes 117 5 5 DISCUSSION OARA E E EEA AE A EET 118 5 5 1 CH radical formation from the photolysis of fresh ASM cecceeeeeeeeees 119 5 5 2 OH radical formation from the photolysis of fresh ASM 0cceceeeeneeeeees 122 5 5 3 Other possible secondary photoprocesses cceceeceec eee eec ene ene ene eneeneenenes 123 5 6 CONCIISLOM eiee a n a a oa bits Wate lode ean ghe hanced E nals a 124 References for Chapter Sse iiccadseeaacwseend eanvanndoass oyesadnes caps iden eeas woke deesaaies 127 Chapter 6 Summary asesan e aE end eae oes ca A E anche alien Catena obs as Sd ad OTR aio 130 6 0 SUMAYE E e ts ean ques E e e E E coub ee acess cee telend EN E doteen E ER 130 Appendix o TEER vee REEE aes OO EA TTE as Nand eee Dee 133 ix List of Figur
96. s In chapter 2 of this thesis an overview of the vast literature on the photochemistry of amorphous solid water and polycrystalline ice with a special emphasis on experimental studies of photoinduced reactions is provided The phases of water ice found in interstellar ices the properties of vapor deposited water ice spectroscopic characteristics of the ice matrix the correlation between growth conditions and structure and effects of photolysis on the ice matrix are presented The VMI apparatus at Queen s University and in particular the ion optics and software components are presented in Chapter 3 The kinetic and angular distributions of desorbed photoproducts can be determined by the back projection of the raw image of photofragments A back projection method based on a Monte Carlo simulation is presented in this section The ability to synchronize the operation of individual instruments in the VMI apparatus will be critical in conducting an automated experiment In Chapter 4 the programming of the instrumental components using LabVIEW 8 6 is discussed Experimental components which were modified or constructed during the course of this work are also briefly described The mechanisms and dynamics of the production of CH and OH from the 157 nm photodissociation of amorphous solid methanol at 90 K serves as a guide as to the data that may be expected Preliminary studies on the photolysis of methanol ice were conducted using a TOF MS appa
97. s chapter concerns the chemical behavior and structure of the ice surface in particular with regards to the spectroscopy and the photolysis of surface ice films This will be followed by a discussion of the photochemistry of water ice and of constituents in ice 2 1 Structure and Formation of water ice The equilibrium structure within which a material crystallizes under given conditions of temperature and pressure is determined by the interaction forces between its molecules The properties of ice have been interpreted by their crystal structure forces between their constituent molecules and their energy levels The crystal structure of ice is formed with water molecules linked to each other so that each proton of one molecule is directed towards a lone pair electron hybrid of a neighboring molecule The oxygen atoms in ice are arranged so that they are at the centre of a tetrahedron with each oxygen atom positioned 2 76 A away from four other oxygen atoms The structure of water rich ice in astrophysical environments is usually not that of the familiar thermodynamically stable hexagonal crystalline polymorph Z found almost exclusively on Earth Rather astrophysical water ice is often observed to be in an amorphous form Temperature is the main factor determining the crystallographic phase of ice grown from the gas phase at low pressures Narten et al reported that there are two evidently distinctive forms of amorphous ice
98. s gt HO i CHOH i 50 2 3 8 3 27 OH i CHOH ads gt H i CH3O i 5 4 14 6 3 28 Considering the translational energies for nascent OH given in Table II it can be inferred that the activation barrier for the hydrogen abstraction reactions from ASM is easily overcome 5 5 3 Other possible secondary photoprocesses Jasper et al have reported the formation of a variety of reactive radicals and molecules such as CH30H CH2 H20 HCOH H2 H2CO H2 CH30 H and CH2OH H through the bimolecular reaction of CH and OH radicals in the gas phase In a study on the ultraviolet photolysis of methane water ice mixtures at 20 K Hodyss et al reported that OH radicals produced from the photodissociation of water react with methane to form CH radicals which in turn recombine with OH to form methanol as given in reaction 3 29 CH 3 i OHG M gt CH3OH ads M 3 29 AH 422 6 kJ mol 123 In addition to these reactions Hodyss et al detected the formation of ethane through the recombination of CH radicals via reaction 3 30 where CH resided in adjacent sites in the solid water matrix CH3 i CH3 i M C2H6 i M 3 30 AH 376 6 kJ mol Since ethane was not detected by Gerakines et al in the ultraviolet processing of solid CHOH at 10 K it can be inferred that reaction 3 30 would only arise if nascent CH3 is less likely to encounter other reactive species such as H OH
99. s which are the PCI interface board and the camera head LabVIEW communicates with the PCI interface board by using the SENNTPCI DLL libraries The PCI board communicates with the camera through a high speed serial link Camera control is performed by sending the 94 Camera Operation Code COC to the camera head Once the COC is uploaded to the camera head exposure commences COC will specify the parameters required to create an exposure These parameters include trigger mode delay from trigger to start exposure region of interest and binning These parameters are included in the front panel of the Imaging VI The procedure used in this VI is as follows Initialize the camera Upload the COC Start the exposure Wait until exposure terminates Retrieve imaging data The front panel allows the user to specify the board number run mode mode of operation and the read out mode The board number inputs the board ID to the VI the run mode controls single or continuous trigger exposure the mode of operation initializes the code and the read out mode specifies if the read out is upright or inverted The CCD temperature should be approximately 13 C and the temperature of the electronics should be maintained at 33 C These temperatures are displayed on the respective thermometers on the front panel The camera controls panel allows the user to specify the programmable controls The Camera Type digital control allow
100. s parallel to the plane of the detector then the resulting photofragments will always possess an axis of cylindrical symmetry In such cases the inverse Abel transform can be used to recover a central slice from the 3D distribution In cases where cylindrical symmetry is not observed forward convolution or basis set methods are employed In the onion peeling program published by Manzhos and Loock the velocity and angular distribution is determined by the back simulation of a raw image with user defined parameters Another approach based on Monte Carlo type simulations is presented below 3 1 Experimental Set Up The velocity map imaging spectrometer consists of a high vacuum chamber a pulsed laser system and a particle detection system The high vacuum chamber is typically evacuated to 10 Torr The vacuum system consists of an ionization chamber and a TOF chamber that are evacuated using two turbo molecular pump Pfeiffer TMU521 and 50 Varian TV701 navigator backed by Edwards and Welch rotary pumps The two chambers are connected by a sliding gate valve Gaseous samples are introduced to the vacuum chamber through a molecular beam valve The molecular beam is expanded to the vacuum chamber with a cylindrical nozzle and is collimated by a 0 5 mm skimmer mounted 50 60 mm downstream from the nozzle The molecular beam valve is powered by a pulse driver General Valve Iota One that is synchronized by a LabVIEW program
101. s the user to select the model of the camera connected The choice of the camera model determines the behavior of the other 95 parameters Horizontal binning combines pixels horizontally and Vertical binning combines pixels vertically for increased sensitivity The four digital controls of ROI Left ROI Right ROI Top and ROI Bottom specify the region of interest The camera information panel displays the specifications such as resolution camera ID CCD type and temperature control The image size status is used to display error codes along with the error out panel The horizontal and vertical size indicators display the number of pixels in the image The image grab can be used to save data to a user specified location The Image 12 bit panel shows each image shot at any given time the Count 12 bit panel shows the sum of several shots and the Image 16 bit panel scales the 0 4096 grayscale values of the 12 bit images up to 0 65535 and displays it as a 16 bit file The front panel for the Imaging VI is given in Figures 29 and 30 4 5 1 Implementation of the Imaging VI The Imaging VI has been programmed using several sub VIs made for SensiCam For example the SCSetBoard VI controls the communication with the board SCSetMode VI sets the mode of operation for subsequent camera function calls the SCGetStatus VI interrogates the camera for configuration and temperature information and the SCGetI
102. surface In addition to the UV initiated reactions of water ice the heterogeneous reactions that take place on water ice particles have attracted considerable attention as well The molecular adsorption on ASW is significant in environmental chemistry For example the release of oxygen atoms into the atmosphere from NO adsorbed on ice is important in the formation of ozone and in the cycling of NO in the Arctic and Antarctic boundary 4 Davis et al reported that the summertime boundary layer over the South Pole layer has elevated levels of NO OH and O3 Nitrate plays an important role in snowpack photochemistry as the precursor for the formation of these oxidizing species as given in reactions 1 14 1 16 NO hv 340 nm OCP NO7 1 14 NO7 hv H gt NO OH HONO 1 15 NO hv OCP NO 1 16 38 L Yabushita et al have detected the formation of the oxygen atom via reaction 1 16 and have reported that these reactions play a central role on air pollution at the South Pole Many CO bearing molecules including simple organic molecules such as H2CO and CHOH have been discovered abundantly in the ASW mantle of dust grains These molecules serve as important precursors to form complex organic molecules They require the chemical processes through surface reactions to produce the observed abundances Since CO exists abundantly on the dust surface these molecules are considered to ev
103. ter prolonged irradiation was eliminated by continually refreshing the ASM surface by intermissive exposure to CHOH vapor This was achieved by opening the pulsed valve to deposit a fresh layer of ASM after each laser shot For the CH 0H H 0 co deposited ice photolysis experiments CH30H H20 1 1 mixed vapor was deposited on the gold substrate The exposure of the CH 0H H O mixture was typically 1500 L The TOF spectra of the CH and OH photo fragments were taken as a function of time delay between photolysis and probe laser pulses The time delay was controlled by a delay generator Stanford Research to allow the observation of the flight time of products The measured TOF spectra S ai t Trans Was fitted to a sum of Maxwell Boltzmann MB distributions Smg each defined by their respective translational temperature Tirans The coefficient a was used to define the relative population of each MB distribution 107 S ait Trans 5 aj SMB t Tans 3 15 Sme t Trans 7 t4 exp mr 2kg Trans t 3 16 where r is the flight distance of the photofragment The MB distribution Pms Ei as a function of translational energy F is characterized by the averaged translational energy lt E gt 2kp Trans Where kp is the Boltzmann constant 4 Pywp E kKpT ans E exp Ev ksTtrans 83 17 The signals are assumed to come from a VUV photoirradiation area with a radius of 6 mm As given in Figure
104. th of IR absorption features in amorphous ice was investigated by Hagen et al They reported that when amorphous ice is deposited at 10 K and warmed up to 130 K the bulk OH stretching band shifted to 3250 cm the line width narrowed the peak intensity increased and the libration frequency increased This was due to the increased strength in the hydrogen bond Upon annealing to 130 K amorphous ice deposited at 10 K undergoes an irreversible transformation to a more ordered form With the annealing process the distance between neighboring O O positions decreases and the molecule reorients to optimize the preferably linear O H O bond angles The smaller O O separations result in strengthened hydrogen bonds Jenniskens and Blake reported that irreversible changes continue to occur in the same parameters when amorphous ice is kept at 140 K for a prolonged time These changes correspond to the transformation from amorphous ice to crystalline ice Ic Since these spectral changes start at temperatures as low as 120 K it has been inferred that the phase change from amorphous ice to crystalline occurs above 120 K The transformation is completed within 45 min 13 2 2 2 Dangling OH A fundamental characteristic of low density amorphous ice is its microporosity Amorphous ice formed below 90 K is a microporous network with a large surface area of approximately 400 m g due to the presence of nanoscale pores Pore widths were measure
105. the initial delay and time base The delay is set with reference to the rising edge from the pulse in counter 0 Total delay for each counter is displayed in the Show Relative Delay wave graph Any errors that occur during operation will be displayed in the Error out panel 4 4 Collecting TOF and REMPI spectra using LabVIEW The following section discusses details of the VI written to collect TOF and REMPI spectra The TOF spectra were collected using an oscilloscope and were transferred to the computer for analysis The REMPI spectrum for each species is then calculated from the TOF spectrum Since the TOF profiles are recorded as a function of the probe laser wavelength a stepper motor unit was utilized to manipulate the probe laser wavelength using the computer Hence the VI described in this section has three panels 89 1 Stepper motor panel to control probe laser 2 Oscilloscope panel to store TOF spectra 3 TOF and REMPI spectra panels Each of these components will be discussed in the following section 4 4 1 Stepper motor panel to control probe laser The ability to control the probe laser wavelength is important in aligning the probe laser and when collecting the TOF spectrum This is achieved through a stepper motor NEMA 17 Unipolar that controls the grating of the laser The stepper motor in turn is controlled via a data acquisition device Measurements Computing USB1208FS that triggers the stepper motor d
106. tion reactions with H atom photoproducts to produce methane CH3 i H i M gt CH i M AH 439 3 kJ mol 3 24 120 The H atoms formed on amorphous ice are known to move between 6 60 A leading to an efficienct recombination process The formation of H atoms on ASM at 90 K following the 157 nm photodissociation through reactions 3 1 and 3 7 has been reported by Hama et al In studies on the ultraviolet processing of ASM at 10 K Gerakines et al detected the IR absorption feature for CH4 but the absorption feature for CH was not detected It can be inferred that this was a consequence of the CH radicals being recombined with the mobile H atoms via reaction 3 24 In the CH30H H20 experiment the intensities of both 3000 and 90 K CH components were reduced This can be attributed to the reduction in the CH OH density on the surface or to the recombination of CH radicals via reaction 3 24 or to the reaction of CH3 radicals with OH radicals produced from the photolysis of H2O The possibility of the reaction between CH radicals and solid water can be eliminated due to the large activation energy requirement in reaction 3 25 CH3 i H O ads CH4 1 OH 3 25 AH 104 6 kJ mol 138 1kJ mol The ratio of the 3000 and 90 K components in the TOF profile of CH products from the mixed ice of CH30H H20 was found to be similar to that from the fresh ASM samples with a dominant f
107. tric v3 modes in the isolated water molecule A broad band is observed at 3370 cm due to the OH stretching mode in the bulk of amorphous ice at 98 K The third band 1665 cm v2 is a result of the bending mode The fourth prominent band observed at 763 cm is a consequence of libration i e the hindered rotation of the water molecule In addition comparatively weak combination bands are observed at 2205 cm 3 v or The IR absorption spectra of H2O adsorbed on Au 111 are given in Figure 2 V2t VL The IR absorption features observed in the water molecule are dependent on the hydrogen bonding of the system The difference between the vibration frequencies of HO molecules in the gas phase and the frequency in unannealed amorphous ice for the bending mode is 70 cm and for the OH stretching mode 450 cm Elevated frequencies in libration and bending modes may be observed due to the presence of a strong hydrogen bonded network Strong hydrogen bonds hinder libration and bending and weaken the normal OH stretching frequencies 11 Reflectance Annealing temperature K l 4000 3000 2000 ot Wavenumber cm FIG 2 Temperature dependence of the IR absorption spectra of H2O adsorbed on an Au 111 surface at 98 K H2O was first adsorbed at 98 K and then the substrate was heated step wisely to the indicated temperature Reproduced from Sato et al 12 The effects of hydrogen bonding on the frequency and wid
108. ugate base of the acid HA and will form a hydrogen bond O H A In this case the hydrogen atom accepting the H2O molecule acts as the base The conjugate base A competes with H20 for the hydrogen atom If the H2O HA bond is stronger than the HOH OH bond the weakening of the bonds in the H2O network causes the OH stretching mode to shift to a lower frequency while gt 16 In the case of weak acids this the frequencies of libration and bending increase process will lead to incompletely hydrogen bonded H2O molecules in the H2O network This incomplete bonding will cause the 3220 cm shoulder to increase Tielens et al observed that the 3220 cm shoulder will disappear if the strength of the hydrogen bonds between H20 and the acid is equal to that between two H20 molecules An incident molecule with the capacity to donate hydrogen atoms as well as accept hydrogen atoms to form hydrogen bonds is characterized as an amphoteric The effect of an amphoteric on the H2O spectrum will be a grouping of the characteristics explained above For example methanol has amphoteric character like water and causes small 22 changes in the H2O spectrum mainly due to the disruption of the hydrogen bonded network 2 3 Amorphous Solid Water The chemical and physical characteristics of amorphous water ice are greatly affected by the microporosity in its structure The total gas absorption area formed in the amorphous solid as
109. ute code repeatedly or conditionally They are comparable to the loops conditional statements and case structures used in text based programming These nodes and sub VIs can also make use of the libraries available for the instrument Terminals in the block diagram serve as entry and exit points for data between the front panel and the block diagram The terminals nodes and sub VIs are connected to each other to perform a task using virtual wire connections Wires serve as data paths between source and destination terminals They are comparable to variables in text based programming languages In text based programming languages different data types are used to eliminate the confusion of variables LabVIEW implements the same principle in graphical programming by using different colors and styles in its wiring Hence the block diagram is a visual representation of the inputs functions sub VIs wired connections and outputs The functionality of a LabVIEW program in comparison to a text based program is illustrated in the following example Let us suppose that a user would like to specify the number of measurements he wants from a certain device and would like to see how many measurements the device has taken while he is running the experiment This repetitive cycle of measurements can be implemented using a for loop in a standard 73 programming language The text based code written for this loop using Visual Basic will be as foll
110. ve devices as shown in Figure 24 In the master device panel the user is able to input the amount of time the pulse must be high and the amount of time in seconds to wait before generating the first pulse by using the digital controls for width and initial delay respectively The initial delay in the master device is defined as To 87 Dev2 ctro Jo c0s000 20 001000 st Dring iDev2 RTSIO I A A S ai A fJo c0s000 fo 002000 fJo o0s000 fo 004000 Devgjcr3 l o 00s000 o 006000 J 1 000000 ffo oso00 gy Devzjetr3 0 005000 o 006000 te We f 0 005000 0 006000 FIG 24 Front Panel of PCI 6602 VI In the slave device panel the counters 1 4 have been assigned to the following devices Counter 1 Nd YAG laser flash lamp Counter 2 Nd YAG laser Q switch Counter 3 DG 535 Counter 4 Pulsed Valve 88 The names of the counters for each of the devices can be selected from the DAQ physical channel constants drop down menu The user is also able to specify the pulse width and initial delay for each counter using the respective digital controls The time base indicator displays the reference for the delay in each counter The default setting for the time base indicator in all the counters has been set to To However the user is able to change this reference using the time base digital indicator The delay indicator displays the calculated total delay for each counter based on
111. vironment The photoproducts which are formed after irradiation of the ice matrix with a UV laser pulse are detected quantum state selectively using a second UV laser through the resonance enhanced multiphoton ionization REMPI technique The nascent photoproducts are then detected by projection on a position sensitive detector and their velocity and angular distributions are recorded This process is called velocity map imaging VMI Using energy and momentum conservation it is then possible to calculate the energy transferred to the ice matrix By characterizing the structural changes that take place in the ice matrix through Fourier Transform Infrared FT IR spectroscopy it is possible to obtain even more detail on the reaction mechanisms between water ice molecules and the photoproducts The determination of kinetic and internal energy distributions of the nascent desorbed species together with the spectroscopic signatures of the ice matrix and the stable trapped photoproducts is expected to provide the complete photochemical mechanism Preliminary work in support of the development of this apparatus has been conducted through a collaborative study involving the author and the Kawasaki Group at Kyoto University using a similar machine albeit without velocity map imaging capabilities The experiments with the Kawasaki group illustrate the capabilities and limits of the state selective detection of photoproducts following methanol ice photolysi
112. w density form between 38 to 68 K and the restrained amorphous form Jar preceding cubic ice Z at 131 K When amorphous ice films are further annealed crystalline ice transfers to polycrystalline cubic ice Z and hexagonal ice The J phase prevalent on earth can also be produced directly by condensation onto a substrate cooled to temperatures above 190 K Condensation below 190 K but above 135 K leads to I If warmed between 160 200 K J will transform irreversibly into J This wide temperature range has been attributed to the dependence of the crystallization temperature on the size of cubic ice crystals These observations imply that the crystallization process which commences just above the glass to liquid transition temperature is incomplete because fragments of the non crystalline microphases co exists with J up to 200 K Amorphous ice is the dominant form at temperatures below 130 K This phase of ice converts to J at a rate that depends on the temperature as given in Figure 1 Jar coexists with from 148 K until 188 K The existence of Jar is responsible for the irregular gas retention and gas release from water rich ices at temperatures above 150 K Figure 1 illustrates the existence of amorphous and crystalline ices at different temperatures FIG 1 Structural dependence of ice on temperature In addition to this Berland et al 1 reported that the density of vapor deposited ice films as a function
113. which differ in density as well as in the second nearest neighbour oxygen oxygen distribution The low density form was estimated to be 0 94 g cm at 77 K and the high density h 1 1 g cm at 10 K The nearest neighbour O O separation was 2 76 A in the low density form The high density form had similar an X ray diffraction pattern similar to low density amorphous ice However it showed an additional peak at 3 3 A Narten et al suggested that the increase in density was a consequence of water molecules occupying the distance between the first and second nearest neighbour at the interstitial sites of the network The additional peak was caused by such water molecules and was the first indication of the occurrence of structural polymorphs of water Low density amorphous ice produced by water vapour deposition below 130 K was determined to be a highly porous open network with a surface area of 150 500 m g This form of ice also has a noteworthy concentration of surface OH groups as observed through the measurements of surface area density of ice films and IR spectroscopy of water ice Furthermore Jenniskens and Blake designed experiments to simulate interstellar ices by observing the structural changes of vapor deposited on water ice in vacuum between temperatures of 15 to 188 K They reported the existence of three amorphous forms and two crystalline forms of water ice High density amorphous water ice is found at 15 K the lo
114. y parameter as inputs The photofragments desorbed from the ice matrix will be ionized using the 2 1 REMPI process and the resulting photofragments are accelerated through the ion optics assembly before being projected on to the MCP The y z coordinates of the photofragments at 59 the detector can be expressed as follows This section is related to the back projection method published by Whitaker FIG 15 Schematic for the projection of ionized photoproducts originating at point O on to the detector As shown in Figure 15 the point of origin of the photofragments which corresponds to the REMPI ionization region is defined as origin x y z 0 The initial velocity distribution of the photofragments can be written using spherical polar coordinates0 and the initial velocity vg as V vo Sin cos oO v v sind sind oy 0 2 8 V Vy COS 0 60 The ionized fragments will then be accelerated through a homogenous electric field E along the TOF axis O with a force of F Eq 2 9 where q is the elementary charge The co ordinates x z at the detector are calculated as y 2heospsin 9 fin sin p singsin 6 2 10 p z LSA sin sin 0 p singsin a 2 11 p where p is the ratio between electrostatic energy acquired in the field and the initial kinetic energy in the particle p ae 2 12 mv 2 and L is the distance between the region where the desorbed products

velocity map imaging apparatus for studies on the photochemistry of

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