Molecular beam studies of HCl interactions with pure and HCl-covered ice surfaces

The dynamics of HCl collisions with ice surfaces is studied using molecular beam techniques. The experiments are carried out with a water vapor pressu re of up to 3 x 10(-5) mbar outside the ice surface, which allows experimen ts to be performed at surface temperatures of 127-180 K. At the higher surf ace temperatures, the ice has a very dynamic character and constantly under goes evaporation and condensation. Angular-resolved intensity and time-of-f light distributions are measured with mass spectrometry, and the effects of surface temperature, incident kinetic energy, and HCl surface coverage are investigated. The dominating outcome of the surface interaction is loss of HCI by sticking to the surface with a residence time of more than 1 ms. Sm all direct scattering and trapping-desorption channels are also observed de pending on the conditions. For a pure ice surface the sticking probability is 1.00 +/- 0.02 at thermal incident kinetic energies, E, while a small dir ect scattering channel is observed when E is increased, reaching a probabil ity of 0.015 +/- 0.005 at E = 0.53 eV. For HCl-covered ice surfaces at 165 K and with thermal incident energies, the sticking probability is 0.88 +/- 0.03 and a trapping-desorption channel (surface residence time less than 30 mu s) with a probability of 0.12 +/- 0.02 is also observed. A direct scatt ering channel opens at higher energies, reaching a probability of 0.08 +/- 0.02 at E = 0.53 eV. For all surface conditions, the collisions are highly inelastic with large energy loss observed for the directly scattered flux, comparable to the results fur the previously studied Ar-ice system.