Dynamics of simple gas–surface interaction. II. Rotationally inelastic collisions at rigid and moving surfaces

Abstract

We have used the classical trajectory method to investigate rotationally inelastic encounters between diatomic molecules and a hierarchy of model surfaces: a rigid surface (RS), a simple harmonic oscillator (SHO), and a generalized Langevin oscillator (GLO). The diatom masses correspond to NO throughout, and the gas–surface interaction potential was invariant, with an attractive potential of ε=0.58 or 0.2 eV. Collision energies were 0.3 or 0.7 eV. Encounters were classified as ‘‘direct,’’ ‘‘indirect,’’ and ‘‘adsorbed.’’ Change from the RS to the SHO surface markedly increased the percentage of indirect encounters; change from SHO to GLO introduced adsorbed trajectories. Rainbow structure in the product rotational distribution, clearly evident on the RS, was obscured by the surface motion in the SHO and GLO models, remaining evident nonetheless for the higher collision energy. Sticking on the GLO surface decreased with increasing initial rotation, particularly for the weaker attractive potential. Consequently application of time-reversal symmetry led to a yield of desorbing molecules weighted toward lower final rotation, i.e., to a rotational temperature TROT<TS (TS=surface temperature). The need for detailed experimental studies to establish the dynamics is evidenced by the fact that the characteristic bimodal distribution over final rotational states is obtained for all three model surfaces—RS, SHO, and GLO.