Rotational state distributions of NO molecules after interaction with germanium surfaces

The rotational state distributions of NO molecules scattered at clean and oxidized Ge surfaces were determined by means of laser induced fluorescence (LIF). For low incident kinetic energies (E/sub kin/< or approx. =200 meV) scattering is dominated by trapping/desorption and the molecules coming off the surface are translationally accommodated to the surface temperature. The rotational state populations can in this case be approximated by a Boltzmann distribution. The rotational temperature T/sub rot/ equals the surface temperature T/sub s/, however only up to about 250 K. Further increase of T/sub s/ leads to a leveling off of T/sub rot/ until a limiting value T/sub rot,lim/roughly-equal400 K is reached. (Rotational cooling in desorption.) At higher kinetic energies direct-inelastic scattering dominates, and for E/sub kin/ = 820 meV a pronounced non-Boltzmann distribution (rotational rainbow) is observed. The observed effects are qualitatively interpreted in terms of current theoretical models