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 pl ane (0001) face of ice I-h at normal and off-normal incidence. The dependen ce 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 deg rees, 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 pe netration of metallic or covalently bonded crystals by atoms becomes possib le. This possibility is due to the open structure of single-crystalline ice I-h, in which the water molecules are arranged in superimposed hexagons, f orming shafts running perpendicular to the ice surface. The penetration mec hanism is operative for the entire range of T-s studied (110-190 K). The se cond sticking mechanism, i.e., adsorption, occurs for all E-i, theta (i), a nd T-i. For theta (i)< 45 degrees, the adsorption probability increases wit h 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 co s(2) theta (i). However, for theta (i)greater than or equal to 45 degrees, the adsorption probability decreases with theta (i). The energy transfer fr om 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 Inst itute of Physics.