Molecular Beam Time of Flight Analysis of the Reaction Dynamics of Alkane Molecules Interacting with a Pt(111) Surface

Molecular beam time-of-flight analysis has been applied to investigate the reaction dynamics of hyperthermal alkane molecules such as methane and ethane interacting with a Pt(111) surface. Below the threshold incident kinetic energy and surface temperature, where direct inelastic collision is dominant, the velocity distributions of scattered methane and ethane molecules qualitatively agree with the prediction based on the classical cube model [R. M. Logan and R. E. Stickney: J. Chem. Phys. 44 (1966) 195.]. Rotational mode excitation is found to be much more significant in the case of ethane molecules than methane molecules. Once dissociation occurs, however, reaction products roughen the surface and reduce the effective mass of the surface, which makes the model no longer applicable. The deviation from the prediction can be understood as a result of increased tangential energy transfer and the multiple collision of incident molecules at the interacting surface.