Time-of-flight measurement of CH4 molecules rainbow scattered from a LiF(001) surface

Time-of-flight spectra of CH4 molecules scattered from a LiF(001) surface h ave been obtained at various scattering angles by making use of a cross-cor relation chopper blade for the [100] and [110] azimuthal directions. The in cident translational energies of CH4 molecules are varied in the 190-500 me V range, while the target surface temperature is maintained at 300 K. The e xperimental results have been examined in relation to the washboard model [ J. C. Tully, J. Chem. Phys. 92, 680 (1990)] which is modified here to take into account the speed distribution of incoming molecules. A qualitative ag reement on their angular dependence has been obtained both in the mean spee d and the energy spread of the scattered CH4 molecules, which reflects the effect of the strong corrugation of the sample surface. Their quantitative differences are partly explained by the translation-rotational excitation o f CH4 molecules during collision. (C) 2000 American Institute of Physics. [ S0021-9606(00)70220-0].