Scattering of Diatomic and Polyatomic Molecules from the (100) Crystal Face of Platinum

The angular distributions of thermal energy molecular beams of diatomic (CO, N2, O2, NO, H2, D2) and polyatomic (CO2, N2O, C2H2, NO2, NH3, and methylene cyclobutane) molecules were monitored following scattering from the (100) crystal face of platinum. All of the gases studied (with the exception of ammonia) yielded directed scattering patterns with the maximum intensity peaked at or near the specular angle for scattering from a clean platinum surface or from a platinum surface covered by a layer of ordered graphite. Broad, cosinelike angular distributions which were independent of incident angle were observed, however, for the cases of C2H2 scattered from an adsorbed layer of C2H2 and CO scattered from adsorbed CO. These results seem to indicate that efficient energy transfer between the surface atoms and gas molecules, as indicated by angular distributions, occurs only in specific instances. Apparently, the incident particles are reflected whenever the nonlocalized vibrational energy modes of surface atoms and the translational energy of the incident molecules are of similar magnitude. The adsorption of gases may lead to the formation of additional low frequency localized surface modes that can be excited by the incident molecules. The efficient transfer of energy via localized surface modes would increase the residence time of the incident molecules and could thus account for the cosinelike angular distributions observed for C2H2 and CO.