A molecular-beam study of the collision dynamics of methane and ethane upon a graphitic monolayer on Pt(111)

Utilizing a supersonic molecular-beam scattering technique, the angular intensity distributions of alkane molecules ( C H 4 and C 2 H 6 ) have been measured, which are scattered from a chemically inert and highly oriented monolayer graphite (MG) on Pt(111). A MG which covers the Pt(111) surface with a full monolayer is found to induce a large energy loss of alkanes during collision with the surface by phonon creation due to the large mass ratio of an alkane molecule with respect to MG. Based on the classical cube model, only applicable to the molecules without internal mode excitation, the effective masses of MG of 76 (six atoms of carbon) and Pt(111) of 585 (three atoms of platinum) are determined from rare-gas atom scattering data. Despite the difference in the degree of freedom between C H 4 and rare-gas atoms, C H 4 scattering is found to be well described by the simple hard-cube model as a result of the high symmetry of the C H 4 structure. With the recently developed ellipsoid-washboard model, an extension of the hard-cube model to include some internal mode excitation of impinging molecules in addition to the surface corrugation, it is found that unlike C H 4 the cartwheel rotation mode of C 2 H 6 is significantly excited during collision, while the helicopter mode excitation is negligible on a flat MG surface.