Unusual angular dependence of trapping probability at highly corrugated surfaces

We show that the kinematics of gas–surface scattering has two distinctively different regions which bear consequence on the incident angle dependence of the gas/surface trapping probability. The two regions are characterized by 1−μ−2λ≳0 and 1−μ−2λ<0, where λ represents the degree of surface corrugation, and μ is the mass ratio of the gas particle to that of the effective surface scattering unit. When 1−μ−2λ≳0, the gas particles after initial scattering have a high probability of reflection. In this case, the trapping probability exhibits normal behavior, i.e., increases with an increase of the scattering angle. For the other case, the gas particles maintain a mean velocity towards the surface after the initial scattering event, and the trapping probability exhibits unusual behavior, i.e., decreases with increasing the scattering angle. A theory constructed for the latter case shows good agreement with experimental data for Cl2 molecule scattering on the Cl covered GaAs (110) surface.