| The exploring of complex reactions within femtosecond resolution provides new challenges because the reactions evolve along multiple pathways. A novel technique, femtosecond-resolved mass spectrometry, is successfully developed to dissect these complex systems into the elementary reactions with temporal, speed, angular and state resolution(s) of the reaction dynamics. With these capabilities, the dynamics and mechanism 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. A variety of dynamic behaviors have been revealed, including product rotational and vibrational excitation, electronic and vibrational predissociation, and saddle-point transition-state dynamics.; A lot of prototype bimolecular reactions were studied. For the first time, the famous electron-donor-acceptor charge-transfer reactions are microscopically resolved and fully understood. Several concepts have been addressed including the reversibility of electron transfer, the nonconcertedness, 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, atom-molecule inelastic collision and four-center covalent-covalent bimolecular reaction were studied by several novel methods and the rate-determining step, the collision complex and the cooperative motion of four centers were observed, respectively.; The solvation ultrafast dynamics were systematically studied from small to lager clusters by monitoring the temporal evolution and translational energy distributions of the escaped solute to elucidate the different solvent structures. Finally, the nonradiative dynamics of big organic molecules, using azines as examples, were studied. 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. |
