STICKING AND SCATTERING OF GAS-PHASE ORIENTED NO AT PT(100) AS A FUNCTION OF THE TRANSLATIONAL ENERGY AND OF MOLECULAR-ORIENTATION

Using a hexapole technique, a pulsed beam of state-selected NO molecul es is focused onto a Pt(100) surface with orientation of the molecular axis so that either preferential N-end or O-end collisions occur with the surface. From the analysis of the partial NO pressure behind the crystal the initial sticking probabilities for oriented as well as for unoriented NO molecules can be determined. The translational energy o f the molecules is varied between 100 and 350 meV by seeding in helium at different NO concentrations. The sticking probability is larger fo r N-end collisions and decreases with increasing translational energy. Likewise, a similar decrease is also observed in the orientation-depe ndent scattering asymmetry. The results show that both direct chemisor ption and trapping into a precursor state occur preferentially for N-e nd collisions.