No abstract

High resolution angularly resolved time of flight distributions are presented for a supersonic argon beam scattering from a clean well-characterized Pt(111) single crystal. A novel presentation of the resulting velocity and angular flux information in terms of iso-flux contour maps in Cartesian velocity space allows the scattering process to be decomposed into three mutually independent directions defined by the surface normal (z), parallel to the surface and in the scattering plane (y), and parallel to the surface but perpendicular to y and z(x). The iso-flux contour maps appear as nested ovals with principal axes oriented parallel to the above defined directions; axis length decreases in the order z, y, x. The corresponding variances in the x, y, and z velocities vary directly with the surface temperature. Three beam energy regimes are evident and are discussed in terms of the diminishing effect of the attractive well which occurs for increasing beam energies and the increasing effect of short range phenomena prevalent at high incident beam energies. Accomodation coefficients were defined and measured for the y and z directions and were 0.1 and 0.45, respectively, indicating the degree to which parallel and perpendicular momenta are not conserved in a single collision. Geometric scattering by instantaneous surface roughness is experimentally shown to be negligible for this system for beam energies <20 000 K indicating that the scattering is by the finite momentum of the surface.

The rotationally inelastic scattering of HD(J = 0→, 1, 2, 3) from a clean Pt(111) surface is reported. Discrete peaks for each final J state are resolved in the in-plane angular distributions, with the inelastic peaks typically being large compared to the elastic peak. Sharp modulations of these inelastic transition probabilities were found as the incident angle was varied, suggesting bound level resonances. These resonances are inconsistent with G-vector mediated (elastic) selective adsorption, but are consistent with rotationally inelastic selective adsorption into the bound levels of the physisorption potential. Observation of bound level resonances can be used to determine molecule–surface potentials. However, unlike elastic selective adsorption, highly probable rotationally inelastic HD selective adsorption is applicable to metallic surfaces of low corrugation.

Velocity distributions for Ar atoms scattering from a clean, polycrystalline tungsten surface have been measured for a wide range of incident supersonic beam energies 300 K < (1/2) m〈v2〉/k<2000 K, and surface temperatures 350 K<Ts<1900 K. This work studies directly the nature of the scattering process of an intermediate mass atom on a clean metal surface over a very wide range of conditions. Direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process. No distinct elastic or quasielastic scattering occurs. Only at the lowest temperatures is a trapping–desorption scattering process observed. The direct inelastic scattering process is characterized by the linear proportional relationship 〈KEe〉=0.83 〈KEi〉+0.20 〈KETs〉 over the entire range of energies and temperatures for 45° angle of incidence and observation in the specular direction (KEe, KEi, and KETs are the kinetic energy of the exiting Ar, the incident Ar, and the Ar in equilibrium at the surface temperature, respectively). The proportionality relationship is nearly the same for observation in the normal direction. The collision-caused spread in velocities is linearly proportional to the surface temperature, and substantially independent of incident energy. Tangential momentum accommodation is also discussed.

The internal state distribution of scattered NO is determined by laser fluorescence excitation spectroscopy when a pulsed, supersonically cooled beam of NO is incident upon the (111) face of a clean Ag single crystal. It is found that the mean rotational energy