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

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