A new method of using quasiclassical trajectories to study the dynamics of elementary reactions is described. Trajectories are initiated in the phase space of a suitably chosen transition state and run forwards and backwards in time from the same starting point to simulate a complete collision. Calculations on a wide range of collinear A+BC reactions involving vibrationally excited reagents reveal that the optimum choice of transition state is a periodic orbiting dividing surface (pods) for which the action over one cycle of the pods is (v+0.5)h The method is extended to three dimensional reactions using the adiabatic periodic reduction scheme to find pods on fixed angle potential surfaces. The complete transition state is defined by joining these pods together. Methods for pseudorandomly sampling the transition state are described and the combined transition state theory-quasiclassical trajectory (TST-QCT) method is applied to the H+H2(v), N+N2(v) and F+H2(v = O) reactions at constant temperature. The TST-QCT method produces relative quantities directly, absolute values are readily obtained using transition state theory. The results of the new method are compared with conventional quasiclassical trajectory studies in the literature. Agreement is very good and the combined method brings about a very great saving in computer time by eliminating trajectories which fail to reach the strong interaction zone as well as revealing the extent of vibrational adiabaticity between reagents and the transition state. Finally, a modification to the TST-QCT method to allow the simulation of fixed collision energy reactions is described and tested on the F+H2 reaction.

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La théorie de la dynamique des processus chimiques élementaires cherche à décrire quantitativement les collisions réactives à l'échelle atomique. Les mouvements des noyaux étant extrêmement difficiles à traiter dans le formalisme quantique, les tomes sont souvent considérés comme des objets classiques. Cepandant, les effets purement quantiques jouent un rôle majeur dans certaines situations, alors que la description classique les néglige. Cette thèse apporte de nouvelles perspectives sur l'inclusion, dans le formalisme clasique, de forts effets quantiques, à savoir la quantification des mouvements internes des réactifs et produits. / The goal of chemical reaction dynamics theory is the quantitative description of reactive molecular collistions at the atomic scale. Since nuclear motions are difficult to study quantum mechanically, nuclei are often considered as classical object. However, quantum effects may play a major role in some situation, and the standard classical description does not take them into account. This thesis brings new perspectives on the inclusion into the classical treatment of one of the strongest qunatum effects, the quantization of reagents and products.

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