Sticking of HCl to ice at hyperthermal energies: Dependence on incidence energy, incidence angle, and surface temperature

We present calculations on the sticking of hyperthermal HCl to the basal plane (0001) face of ice I-h at normal and off-normal incidence. The dependence of the sticking probability on the incidence energy (E-i), the angle of incidence (theta (i)), and the surface temperature (T-s) is discussed. Two sticking mechanisms are observed. For theta (i)less than or equal to 30 degrees, penetration of the (0001) face is possible at an energy of about 100 kJ/mol, which is an order of magnitude lower than energies for which the penetration of metallic or covalently bonded crystals by atoms becomes possible. This possibility is due to the open structure of single-crystalline ice I-h, in which the water molecules are arranged in superimposed hexagons, forming shafts running perpendicular to the ice surface. The penetration mechanism is operative for the entire range of T-s studied (110-190 K). The second sticking mechanism, i.e., adsorption, occurs for all E-i, theta (i), and T-i. For theta (i)< 45 degrees, the adsorption probability increases with theta (i) as would be expected, because the normal component of E-i that needs to be transferred to the surface for sticking to occur scales with cos(2) theta (i). However, for theta (i)greater than or equal to 45 degrees, the adsorption probability decreases with theta (i). The energy transfer from HCl to the ice surface and the energy dissipation within the surface are found to be fast and efficient at normal incidence. (C) 2001 American Institute of Physics.