Laser investigation of the dynamics of molecule–surface interaction: Rotational and translational energy of scattered molecules

Combining molecule surface scattering with state-selective laser-induced fluorescence and laser resonance ionization the internal energy distributions and state- and angular-resolved velocity distributions of surface-scattered NO molecules have been measured. In the experiments a supersonic NO beam was scattered from graphite and platinum surfaces. The parameters being varied were surface temperature and average velocity of the incoming molecular beam. The NO/Pt system shows a trapping/desorption behavior for all surface temperatures and a full rotational accommodation at low and only incomplete rotational accommodation at high temperatures. In the case of the weakly inelastic scattering of NO/graphite a similar behavior of the rotational energy of the scattered molecules was observed. The angle- and state-resolved velocity distributions show at low surface temperatures the coexistence of a quasispecular and an isotropic part. The diffusively scattered NO molecules gain rotational energy, but transfer much more translational energy to the surface, so that the energy balance is considerably negative. For the specularly scattered molecules energy loss and gain are balanced at low surface temperatures. With increasing temperatures the scattered molecules may gain translational energy, but the rotational energy stays constant, resulting under certain conditions in an energy transfer from the surface to the scattered molecules.