Reaction Dynamics of Atomic Chlorine with Vibrationally Excited Methane : Oriented Photofragments in Molecular Photodissociation

다국어 초록 (Multilingual Abstract)

Part I of this thesis presents the investigation of the reaction of atomic chlorine (Cl^(2)P_(3/2)) with the vibrationally excited methane (CH₄), using the photo-initiated reaction technique. A mixture of the Cl₂, CH₄and He is supersonically expanded into a vacuum chamber, and the CH₄reagent is vibrationally excited by a resonant IR radiation. The reaction is initiated by the photodissociation of the Cl₂and the reaction products (HCl and CH₃) are state-selectively detected by 2+1 resonanceenhanced multiphoton ionization (REMPI) in a Wiley-McLaren time-of-flight (TOF) spectrometer. The product state distributions, spatial anisotropies and the stateresolved differential cross sections are measured. The main focus of this work is on understanding of the effects of the different modes of vibrational excitation of methane on reaction dynamics.
It is found that the stretch-excitation of C-H bond leads exclusively to H-atom abstraction for partially deuterated methanes. In addition, two different modes of stretch-overtone excitation of methane (｜11〉：one quantum in each of two different C-H stretches,｜20〉：two quanta in one C-H stretch) leads to different vibrational states of the methyl radical product. These observations indicate that the H-atom abstraction reaction proceeds as if the rest of the methane molecule plays only the role of spectator.
The energy disposal and angular distributions for the reaction Cl + CH₄(｜1100;F₂〉) (one quantum each in two of the four C-H bonds;｜11〉) are also studied in detail. Spatial anisotropies of HCl and CH₃products allow identification of three major product channels. They are in order of importance: (a) HCl (v = 0) + CH₃[ν₁(symmetric stretch) or ν₃(asymmetric stretch)=1]; (b) HCl (v=1) + CH₃[ν₂(umbrella bend)=1)]; and (c) HCl (v=1) + CH₃(ν₁=1). The CH₃(v=0) product cannot be detected, and the HCl (v=2) product is minor. Channels (a) and (c) proceed in a vibrationally adiabatic manner, whereas channel (b) appears to involve the nonadiabatic interaction involving the low frequency bending mode in methane that correlates to the bending mode in the methyl radical product. The angular distributions differ markedly for the three product channels. This behavior is explained by the propensity for reactive collisions involving H-atom transfer along the line of centers and the difference in the cones of acceptance. The rotational angular momentum vector of the HCl (v = 1, J = 1) product is aligned perpendicular to the line of centers, which is consistent with an impulsive energy release along the line of centers.
The dynamics of the Cl + CHD₃｜2000;A1〉reaction is compared to the dynamics of the Cl + CH₄｜1100; F2〉reaction. The observation of a substantial amount of HCl (v=2) product from the｜20〉-mode enhanced reaction provided us with the
complementary evidence of the direct and localized nature of the C-H stretch excited reaction. The angular distributions and the rotational distributions of HCl products indicate that the HCl (v=2) product channel has a restricted cone-of-acceptance
for the reaction, whereas the HCl (v=1) channel has a substantially wider cone of acceptance.
The reaction of Cl atoms with CH₄(ν2 +ν4) is studied in order to understand the role of bending-mode excitation. We have established that this reaction is at least 10 % as reactive as the ν₃-driven reaction. We find no pronounced propensity for
the formation of CH₃(ν₂= 1), in contrast to the theoretical predictions from the adiabatic correlation of the vibrational modes of the CH₄and CH₃. The HCl(v=0) products show broad side and backward scattering. Both of the HCl and the CH₃
products show substantial rotational excitations. It is believed that the product rotational excitation is caused by the tangential motion of C-H bond mapped onto the rotational motion of the products.
Relative reactivities of the ground [Cl(^(2)P_(3/2))] versus spin-orbit excited state Clatom [Cl^(*)(^(2)P_(1/2))] with the CH₄(v=0) is studied using the BrCl molecule as a precursor for the Cl and Cl^(*). Measured spatial anisotropy of the CH₃product indicates the Cl^(*) + CH₄reaction channel is unimportant in the near-threshold collision energy range of 0.13 - 0.16 eV, whereas the reaction with ground-state Cl atoms with CH₄excited with one quantum in the ν₂(torsion) or ν₄(bending) mode is dominant.
Part II of this thesis presents the study of the oriented photofragments in the photodissociation of simple molecules. Electronic orbital orientation of the atomic photofragments is caused by the quantum mechanical coherence between the two
pathways that originate from a mixed parallel and the perpendicular transition. An extended formalism is developed for the photofragments (atomic and diatomic) from the photodissociation of bent triatomic molecules. We show that the the broken symmetry associated with a bent triatomic molecule causes both coherent and incoherent orientation of the photofragments.
Orientation moments of the Cl^(*)(^(2)P_(1/2))-atom photofragments from the photodissociation of the molecular chlorine (Cl₂) are studied in the wavelength range 270-400 nm, with linearly polarized light. The orientation of the excited-state chlorine atom Cl^(*)(^(2)P_(1/2)) is probed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) using circularly polarized light. The degree of orientation of the Cl^(*) photofragment is found to oscillate as a function of photolysis wavelength. The measured orientation is caused by the interference between the adiabatic pathway of the B³Ⅱ(□)-X¹∑(□)
transition and the nonadiabatic pathways that involve the C¹Ⅱ(□)-X¹∑(□)transition followed by the nonadiabatic radial-derivative coupling to other excited states with Ω= 1_(u) symmetry.
Orientation moments of the S (^(1)D_(2)) and CO (X¹∑^(+)) from the photodissociation of carbonyl sulfide (OCS) are studied at 223 nm, with linearly polarized light. Orientation moments of the S(^(1)D_(2)) and co(X¹∑^(+)) are probed by (2 + 1) resonance enhanced multiphoton ionization (REMPI) using circularly polarized light. Fast S atoms show a large orientation, whereas slow S atoms show little or no orientation. Orientation of S-atom results from quantum mechanical coherence associated with mixed parallel ^(1)A´(^(1)△)-^(1)A´(^(1)∑^(+)) and perpendicular ^(1)A˝(^(1)∑^(-))-^(1)A´(^(1)∑^(+)) transitions that lead to the same photofragment state S(^(1)D_(2)). Comparison of the speed-dependent orientation with the expected envelope of the oscillation of the orientation suggests that the asymptotic phase differences of the two wave functions are nearly constant over different rotational states of the CO photofragment. This result can be explained by the similarity of the two potential energy surfaces involved. Measured orientation of the CO photofragments oscillates between clockwise and counter-clockwise directions as a function of the rotational quantum number of CO. In addition, use of circularly polarized photolysis induces a lab-frame A_(0)^((1)) 0 moment, which cannot be explained by a simple recoil picture for the photodissociations. Three different mechanisms are proposed to explain the observed trends in the orientation of CO photofragments.

목차 (Table of Contents)

• Contents = x
• Abstract = iv
• Acknowledgments = viii
• Part I. Reaction Dynamics of Atomic Chlorine with Vibrationally Excited Methane = 1
• Chapter 1. Introduction = 2
• 1.1 Vibrational Motion and Reactivity = 2
• 1.2 Motivations and Background = 5
• 1.3 Outline = 7
• Chapter 2. Experimental = 10
• 2.1 Overview = 10
• 2.2 Motivation for the Technique = 11
• 2.3 Principles of the Technique = 12
• 2.3.1 Speed Distribution and the DCS = 12
• 2.3.2 Spatial Anisotropy of the Product = 15
• 2.3.3 Measurement and the Analysis = 16
• 2.4 Experimental Setup = 18
• 2.4.1 Time-of-Flight Spectrometer = 18
• 2.4.2 Tunable Infrared Source = 20
• 2.5 Gas Delivery = 23
• 2.6 Experimental Procedures = 24
• Chapter 3. Spectroscopy: Reagents and Products = 26
• 3.1 Vibrational Spectroscopy of CH₄= 26
• 3.2 2 + 1 REMPI Spectroscopy of HCl and CH₃= 27
• 3.2.1 HCl = 27
• 3.2.2 CH₃= 28
• Chapter 4. Vibrational control with stretch-excitation = 30
• 4.1 Introduction = 30
• 4.2 Methane Vibration near 6000 cm^(-1) = 31
• 4.3 Experimental = 32
• 4.4 Results = 32
• 4.5 Discussion = 34
• 4.5.1 Localized Reactivity = 34
• 4.5.2 Validity of the Local Mode Picture = 36
• 4.6 Conclusion = 39
• Chapter 5. Reaction of CH4 ｜1100; F₂〉= 41
• 5.1 Introduction = 41
• 5.2 IR Spectroscopy of Methane near 6,000 cm^(-1) = 42
• 5.3 Energetics = 42
• 5.4 Rotational polarization of the HCl(v=1,J=1) = 44
• 5.5 Experimental = 44
• 5.6 Results = 45
• 5.6.1 HCl and CH₃Product State Distributions = 45
• 5.6.2 Speed distributions and the spatial anisotropies of CH₃and HCl products = 48
• 5.6.3 State-Resolved Differential Cross Sections = 53
• 5.6.4 Rotational Polarization of the HCl (v=1, J=1) Product = 54
• 5.7 Discussion = 55
• 5.7.1 Rotational Excitation of the Products = 55
• 5.7.2 Product Channels = 58
• 5.7.3 Channel-specific Differential Cross Sections and the Impulse Release = 60
• 5.7.4 Rotational Polarization of the HCl (v=1, J=1) Product = 65
• 5.8 Conclusion = 67
• 5.9 Appendix: Cone-of-Acceptance = 68
• 5.9.1 Near-threshold, Collision Activated Reaction = 68
• 5.9.2 Vibrationally activated reaction = 69
• Chapter 6. Reaction of CHD₃｜2000;A₁〉= 71
• 6.1 Introduction = 71
• 6.2 Energetics = 72
• 6.3 Experimental = 73
• 6.4 Results = 73
• 6.4.1 Internal Excitation of Products = 73
• 6.4.2 Speed Distributions and Spatial Anisotropies of the CD₃and HCl Products = 75
• 6.4.3 Differential Cross Sections for the HCl products = 81
• 6.5 Discussion = 82
• 6.5.1 Vibrational excitation of the HCl product = 82
• 6.5.2 Rotational Excitation and the Angular Distribution = 83
• 6.6 Conclusion = 86
• Chapter 7. Effect of Bending-Excitation = 87
• 7.1 Introduction = 87
• 7.2 Experimental = 89
• 7.3 Results = 89
• 7.3.1 Enhancement upon ν₂+ ν₄Excitation = 89
• 7.4 Internal Excitation of the Products = 93
• 7.5 Differential Cross Sections = 95
• 7.6 Discussion = 96
• 7.6.1 Enhancement of the Reactivity = 96
• 7.6.2 Rotational Excitation of the Products = 97
• 7.6.3 Differential cross sections = 98
• 7.7 Conclusion = 99
• Chapter 8. The effect of spin-orbit interaction = 100
• 8.1 Introduction = 100
• 8.2 Experimental = 103
• 8.3 Results and Discussions = 104
• 8.3.1 Spatial anisotropy of BrCl = 104
• 8.3.2 Speed distribution and β_(phot) of the CH₃product = 105
• Part II. Oriented Photofragments in Molecular Photodissociation = 111
• Chapter 9. Introduction = 112
• 9.1 Photodissociation Dynamics = 112
• 9.2 Motivations and Outlines = 117
• Chapter 10. Oriented photofragments: formalism = 119
• 10.1 Introduction = 119
• 10.2 Physical Origin of Oriented Photofragments = 121
• 10.2.1 Coherent Atomic Orbital Orientation = 121
• 10.2.2 Orientation from Nonlinear Triatomics = 125
• 10.3 Extended Formalism for Triatomics = 128
• 10.3.1 Excited State Wave Function = 128
• 10.3.2 Transition Dipole Vector = 130
• 10.3.3 Density Matrix = 132
• 10.3.4 Detection of the BC-fragment = 134
• 10.3.5 Detection of the A-fragment = 139
• 10.4 Summary = 140
• Chapter 11. Photodissociation of Molecular Chlorine = 141
• 11.1 Introduction = 141
• 11.2 Experimental = 144
• 11.3 Results = 145
• 11.3.1 Im[a₁^((1))(Ⅱ,ㅗ)] Moment = 145
• 11.3.2 a_(0)^((1))(ㅗ) Moment = 148
• 11.4 Discussion = 148
• 11.5 Conclusion = 151
• Chapter 12. Photodissociation of OCS: Part-I = 153
• 12.1 Introduction = 153
• 12.2 Experimental = 155
• 12.3 Results = 157
• 12.3.1 Spatial Anisotropy = 157
• 12.4 Speed Dependent Im[a₁^((1))(Ⅱ,ㅗ)]Moment = 159
• 12.5 Discussion = 162
• 12.6 Conclusion = 165
• Chapter 13. Photodissociation of OCS: Part-II = 166
• 13.1 Introduction = 166
• 13.2 Experimental = 167
• 13.3 Results = 169
• 13.3.1 Im[a₁^((1))(Ⅱ,ㅗ)]Moments = 169
• 13.4 A_(0)^((1))Moment = 172
• 13.5 Discussion = 173
• 13.5.1 Comparison of the Atomic and Diatomic orientations = 173
• 13.5.2 Interpretation of Im[a₁^((1))(Ⅱ,ㅗ)] and A_(0)^((1))Moments = 175
• 13.6 Conclusion = 178
• Chapter 14. Appendix = 179
• 14.1 Sensitivity Factor, s_(k) for the Orientation and Alignment = 179
• 14.2 Definition of Absolute Handedness of the Circularly Polarized Light = 181
• Bibliography = 184