Trapping dynamics of ethane on Si(100)-(2x1): Molecular beam experiments and molecular dynamics simulations

The trapping probability, or physical adsorption probability, of ethane on a clean Si(100)-(2x1) surface has been measured as a function of the incide nt translational energy and incident polar angle of the molecule at a surfa ce temperature of 65 K. At all incident angles the trapping probability dec reases as the translational energy of the incoming ethane molecule is incre ased from 0.05 to 1.3 eV. As the incident polar angle, with respect to the surface normal, is increased, the trapping probability decreases. This decr ease in trapping probability with increasing polar angle contradicts the id ea of normal energy scaling and has been seen in very few cases. Classical molecular dynamics calculations have been employed to study the cause of th is unusual angular dependence. This simulation predicts trapping probabilit ies in good agreement with the experimental data. Analysis of the computed trajectories indicates that the initial site of impact within the unit cell , as well as energy exchange on initial impact with the surface, is importa nt in determining the fate of an incident molecule. Normal momentum of the incident molecule is dissipated during the first impact much more efficient ly than is parallel momentum. The simulations also indicate that the observ ed angular dependence can be explained in terms of parallel momentum accomm odation. Large amounts of parallel momentum remaining after initial impact may be converted to normal momentum on subsequent impacts, causing molecule s to scatter from the surface. Therefore, molecules that impact the surface at glancing angles and high translational kinetic energies are more likely to scatter from the surface than those at normal incidence or with lower t ranslational kinetic energy. (C) 1999 American Institute of Physics.