VELOCITY DISTRIBUTION MEASUREMENTS OF A SUPERSONIC METHANE MOLECULAR-BEAM REFLECTED FROM A PT(111) SURFACE AT VARIOUS REFLECTION ANGLES

Velocity distribution measurements of a supersonic molecular beam of m ethane molecules reflected from a Pt(111) surface have been made and r esults are analyzed based on the classical collision model, the hard c ube model [J. Chem. Phys. 44 (1966) 195]. The ''drift speed'' and ''be am temperature'', deduced by fitting the obtained distributions with t he model calculations, vary with the angle of reflection as predicted by the model. Disagreements are supposed to arise from the simplicity of the model such as neglect of surface potential roughness and the di spersive forces acting between the methane molecules and the surface a toms. Excitation of internal modes of methane molecules during the col lision at the surface is considered possible.