Femtosecond photodissociation dynamics of alkali halides

Femtosecond transition-state spectroscopy (I. S) is a time-resolved spectroscopic technique by which the real time dynamics of the transition state of a chemical reaction can be studied with femtosecond resolution. This technique has been applied to study a number of chemical reactions. This thesis presents one such system, the photodissociation of alkali halides, and that of NaI in particular. Upon excitation to the covalent state, the alkali halide molecule will encounter a curve crossing during its way to dissociation. This curve-crossing phenomenon manifests itself in the dynamics of the photodissociation process. Experimentally, wave-packet trapping and leaking that are due to the curve crossing are observed. Taking full advantage of the femtosecond resolution offered by the FTS technique, the wave-packet dephasing dynamics is separated from the population decay. The Landau-Zener coupling-matrix element is also obtained from the time-resolved data, which is in close agreement with previous literature values. Furthermore, the combination of the temporal resolution and the spectral resolution of the FTS methodology allows the wave-packet motion to be monitored at a specific internuclear separation, which covers a wide range of distances. This feature of the FTS technique makes the mapping of reaction trajectories possible. A classical mechanics-based inversion procedure is then developed to obtain the relevant potential- energy curves involved in the photodissociation reaction from the results of such trajectory mapping experiments. The potentials inverted this way are tested by quantum dynamical simulation and compared with those obtained by other workers from different techniques. The long-time behavior of the wave-packet motion after many oscillations is also discussed