STRUCTURAL AND VIBRATIONAL KINETICS BY TIME-RESOLVED GAS ELECTRON-DIFFRACTION - STOCHASTIC APPROACH TO DATA-ANALYSIS

Time-dependent theoretical intensity expressions are presented which can be used to model time-resolved gas electron diffraction (GED) data of laser-excited species in both nonequilibrium and equilibrium ensembles. The new expressions are needed because the traditional equation used in GED studies is restricted to quasirigid molecules in equilibrium distributions, while a previous formalism developed to study the structural kinetics of coherent states in laser-induced photoexcitation(1,2) requires detailed spectroscopic information on the potential functions of both the ground and excited states of the systems studied. In contrast, the new and different approach presented in this paper can be applied when such information is not available or limited, because the requisite parameters for modeling the GED intensities are directly obtained from refinements of the diffraction data. Simplifying constraints can be introduced in the new formalism which depend on the form of the probability density function assumed for the molecular ensemble. Specific expressions are given for the Boltzmann and Poisson distributions. The former applies to ensembles at equilibrium but is not restricted to small-amplitude motion. The latter is applicable to nonequilibrium distributions. Various model studies were performed with the new formulas, and the results are discussed in detail.