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_kin ≲ 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_rot equals the surface temperature T_s, however only up to about 250 K. Further increase of T_s leads to a leveling off of T_rot until a limiting value T_rot,lim ≈400 K is reached. (Rotational cooling in desorption.) At higher kinetic energies direct-inelastic scattering dominates, and for E _kin=820 meV a pronounced non-Boltzmann distribution (rotational rainbow) is observed. The observed effects are qualitatively interpreted in terms of current theoretical models.