Chemical Reaction Dynamics and Coincidence Imaging Spectroscopy

Abstract

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.