Femtosecond molecular dynamics of complex reactions

Citation

Zhong, Dongping (1999) Femtosecond molecular dynamics of complex reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7GR2-YW91. https://resolver.caltech.edu/CaltechETD:etd-04072008-130657

Abstract

The exploring of complex reactions within femtosecond resolution provides new challenges because the reactions evolve along multiple pathways. New techniques are needed to dissect these complex systems into the elementary reactions. In this thesis, femtosecond-resolved mass spectrometry is successfully developed to achieve temporal, speed, angular and state resolution(s) of the reaction dynamics. With these capabilities, the mechanism and dynamics of the complex reactions are able to be microscopically elucidated for each elementary step by monitoring the temporal evolution of the transition state and final products, measuring the energy deposition among the translational and internal motions of the final products, and resolving the correlation between the structural changes and the energy release.

The complex reactions studied range from unimolecular dissociation, to bimolecular reactions, to cluster solvation, and to nonradiative dynamics. For unimolecular reactions, the level of complexity varies from diatomics to polyatomics, from direct-mode to complex-mode, from one-bond breakage to multi-bond fission. The six different systems were examined and a variety of dynamic behaviors have been revealed, including product rotational and vibrational excitation, electronic and vibrational predissociation, and saddle-point transition-state dynamics.

Many kinds of bimolecular reactions were studied, consisting of the major contributions of this thesis. For the first time, the famous electron-donor-acceptor charge-transfer reactions are fully understood. More than ten systems were examined with the different charge-transfer characters and all elementary steps involved are microscopically resolved. Several concepts have been addressed including the reversibility of electron transfer, the nonconcertedness of reverse electron transfer and the bond breakage, the energy dissipation, and the reaction coherence. The reversible electron transfer is found as a general reaction mechanism. The system has been considered as a benchmark textbook example and provides insights into biological charge-transfer processes.

The aromatic nucleophilic substitution reaction was studied. The reaction is involved with many elementary steps and the rate-determining step was identified. The first atom-molecule inelastic collision was clocked in fs resolution and a novel method is presented by femtosecond detachment. The collision complexes are observed and quantum resonance may play a role. The first four-center covalent-covalent bimolecular reaction was studied and the cooperative motion of four centers was observed. The method presented is general and very important, and should be applied to study a variety of covalent-covalent bimolecular reactions.

The solvation ultrafast dynamics were systematically studied for many systems from small to lager clusters by monitoring the temporal evolution and translational energy distributions of the escaped solute. Different solvent structures are identified based on the dramatic temporal behaviors of the exited solute. Finally, the nonradiative dynamics of big organic molecules, using azines as examples, were studied. Different isomers of the valance-bond azines are observed. The conical intersection was found to play a key role on the ultrafast nonradiative dynamics and seems to be a general phenomenon, consistent with the recent ab initio predictions.

Thesis Files