Trajectory studies of vibrational energy transfer in gas--surface collisions

We have applied the stochastic trajectory method to the scattering of NO from the (001) surface of an LiF crystal. The surface was represented by a 32 atom primary zone and the effects of the rest of the surface atoms were included using the Generalized Langevin Equation method. The forces between the surface atoms were treated using a shell model and the gas--surface interactions included short-range repulsions, attractive dispersion forces, and the interaction of the dipole moment of NO and the surface ions. The dependence of the dipole moment and the dependence of the molecular polarizability on the internuclear separation of NO were also included. The calculated vibrational energy accommodation coefficient was less than 1% when trapping of the gas diatom was negligible. By modifying the vibrational frequency of the scattered diatom we have examined the dependence of vibrational energy transfer on frequency. An exponential dependence on the ratio of the molecular frequency to the surface Debye frequency was found, similar to behavior observed for vibrational relaxation in condensed phases. We have also examined the relative efficiency of the coupling of the vibrational mode to the translational and rotational modes of the gas diatom and to the phonon modes of the surface. We found that the vibrational mode is most strongly coupled to the surface phonons with a much weaker coupling to the rotational and translational modes