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Chemical reaction dynamics and coincidence imaging spectroscopyLee, Anthony M. D., 1976- 05 July 2007 (has links)
This thesis describes and develops two experimental techniques, Time Resolved Photoelectron Spectroscopy (TRPES), and Time Resolved Coincidence Imaging Spectroscopy (TRCIS), to study ultrafast gas phase chemical dynamics. We use TRPES to investigate the effects of methyl substitution on the electronic dynamics of the simple alpha, beta-enones acrolein, crotonaldehyde, methylvinylketone, and methacrolein following excitation to the S2 state. We determine that following excitation, the molecules move rapidly away from the Franck-Condon region reaching a conical intersection promoting relaxation to the S1 state. Once on the S1 surface, the trajectories access another conical intersection leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to explain the variations in relaxation behaviour. We propose a model that uses dynamical factors of specific motions in the molecules to explain the differing nonadiabatic S1/S0 crossing rates.
The second part of this thesis examines the issues involved with design and construction of a Coincidence Imaging Spectrometer. This type of spectrometer measures the 3-dimensional velocities of both photoelectrons and photoions generated from probing of laser induced photodissociation reactions. Importantly, the photoelectrons and photoions are measured in coincidence from single molecules, enabling measurements such as recoil frame photoelectron angular distributions and correlated photoelectron/photoion energy maps, inaccessible using existing techniques. How to optimize the spectrometer resolution through design, tuning, and calibration is discussed. The power of TRCIS is demonstrated with the investigation of the photodissociation dynamics of the NO dimer. TRPES experiments first identified a sequential kinetic model following 209nm excitation resulting in NO(X) (ground state) and NO(A) (excited state) products. Using TRCIS, it was possible to measure time resolved vibrational energy distributions of the products, indicating the extent of vibrational energy redistribution within the dimers prior to dissociation. Recoil frame photoelectron angular distributions and theoretical support allow identification of a previously disputed intermediate on the dissociation pathway. / Thesis (Ph.D, Chemistry) -- Queen's University, 2007-04-01 10:12:39.968
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Chemical Reaction Dynamics at the Statistical Ensemble and Molecular Frame LimitsClarkin, OWEN 12 September 2012 (has links)
In this work, experimental and theoretical approaches are applied to the study of chemical reaction
dynamics. In Chapter 2, two applications of transition state theory are presented: (1) Application of
microcanonical transition state theory to determine the rate constant of dissociation of C2F3I
after π∗ ← π excitation. It was found that this reaction has a very fast rate constant
and thus
is a promising system for testing the statistical assumption of molecular reaction dynamics. (2) A general
rate constant expression for the reaction of atoms and molecules at surfaces was derived within the statistical
framework of flexible transition state theory.
In Chapter 4, a computationally efficient TDDFT approach was found to
produce useful potential energy surface landscapes for application to non-adiabatic predissociative dynamics
of the molecule CS2 after excitation from the ground state to the singlet C-state. In Chapter 5, ultrafast
experimental results of excitation of CS2 to the predissociative neutral singlet C-state is presented. The
bandwidth of the excitation laser was carefully tuned to span a two-component scattering resonance with each
component differently evolving electronically with respect to excited state character during the quasi-bound
oscillation. Scalar time-resolved photoelectron spectra (TRPES) and vector time-resolved photoelectron
angular distribution (TRPAD) observables were recorded during the predissociation. The TRPES yield of
photoelectrons was found to oscillate with a quantum beat pattern for the photoelectrons corresponding to
ionization to the vibrationless cation ground state; this beat pattern was obscured for photoelectron energies
corresponding to ionization from the vibrationally excited CS2 cation. The TRPAD data was recorded for
two general molecular ensemble cases: with and without a pre-excitation alignment laser pulse. It was found
that in the case of ensemble alignment (Chapter 6), the “molecular frame” TRPAD (i.e. TRMFPAD) was
able to image the purely valence electronic dynamics of the evolving CS2 C-state. The unaligned ensemble
TRPAD observable suffers from excessive orientational averaging and was unable to observe the quantum
beat.
Engineering efforts were also undertaken to eliminate scattered light background signal (Chapter 7,
Appendix A) and improve laser stability as a function of ambient pressure (Appendix B) for TRMFPAD
experiments. / Thesis (Ph.D, Chemistry) -- Queen's University, 2012-09-11 22:18:20.89
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