Femtosecond reaction dynamics in the gas phase

The advent of short pulsed laser sources has allowed time-resolved methods to be developed by which molecular dynamics can be studied directly. This facilitates the observation of processes that may not be amenable to study by frequency-resolved techniques. This is particularly so in the case of reaction dynamics, where many changes may be occurring in a short period of time and the transition state is too short-lived to be readily observed by continuous wave (cw) methods. The work described here involves the study of two such processes in the gas phase by femtosecond pump-probe spectroscopy.; The first involves photoinduced molecular detachment of halogens from gem-dihaloalkanes using a 312 nm femtosecond pulse. The progress of the reaction is monitored by selective detection of fluorescence from the halogen product. The reaction was found to be general to a number of dihaloalkanes, and in every case to produce the appropriate molecular halogen product in the D ′ state. Molecular dynamics were probed by fluorescence depletion using a femtosecond pulse at 624 nm. Vibrational coherence was observed in some halogen products, indicating a concerted reaction mechanism. Analysis of the spectroscopic and dynamic data was performed; it was determined that for one molecular detachment channel the reaction proceeds by an asynchronous concerted mechanism.; The second experiment is a real-time study of an unrestricted bimolecular reaction. In this process, pairs of gas phase mercury atoms are photoassociated to an electronically excited state using a femtosecond pulse. The dynamics of the resulting molecules are probed by fluorescence depletion using a second pulse. Analysis of the rotational anisotropy in the nascent dimers conclusively confirms that dimer formation is photoassociative. The degree of rotational excitation in the nascent molecules indicates the impact parameter selectivity of the photoassociation process.