STRUCTURAL KINETICS BY STROBOSCOPIC GAS ELECTRON-DIFFRACTION .2. TIME-DEPENDENT MOLECULAR INTENSITIES OF PREDISSOCIATION PROCESSES

Various attempts have been reported in the literature [1-3] to develop stroboscopic gas electron diffraction (GED) for time-resolved studies of dynamic phenomena in laser-excited molecular species. This novel a pplication of GED requires the formulation of new expressions for the scattering intensities, since conventional equations for GED data anal ysis are based on models that are restricted to equilibrium ensembles and molecules exhibiting small amplitude motions. As a first example, the time-dependent molecular intensities for species in adiabatic diss ociative states have recently been reported [4]. In the current study the molecular intensity expression for the evolution of internuclear d istances in predissociation processes on a diabatic potential energy s urface is presented and some model radial distribution curves are show n. The structural kinetics of the photodissociation of NaI are modeled on a femtosecond time-scale. The time-dependent molecular intensities of NaI are directly parameterized in terms of the intramolecular pote ntial energy surfaces and other parameters which characterize the tran sient state dynamics, as taken from experimental spectroscopic investi gations reported in the literature. The acute dependence of the molecu lar intensities and radial distribution curves on the PES shows that s olution of the inverse problem, i.e. determination of potential parame ters from time-dependent electron diffraction data, is feasible in pri nciple. As in the previous model study of adiabatic dissociative proce sses, it is found that details of the dynamics of photo-predissociatio n can be resolved on a time-scale shorter than that of the probing ele ctron pulse. Furthermore, our analysis demonstrates the fundamental di fference between the observables obtained from spectroscopy and electr on diffraction: whereas no explicit structural information on the time -evolution of the nuclear configuration is obtained from spectroscopy, the molecular intensities of scattered electrons depend directly on t he structural kinetics; i.e. the time-evolution of the internuclear di stances.