Dynamics of argon collisions with water ice:: Molecular beam experiments and molecular dynamics simulations

The dynamics of argon atom collisions with water ice at 110-180 K is investigated using molecular beam;experiments and molecular dynamics simulations. Initial argon energies of 0.065 to 0.93 eV and incident angles of 0 to 70 degrees are used, and directly scattered and thermally desorbed atoms are separated by angular-resolved time-of-flight measurements. For thermal incident energies the scattering is almost entirely due to trapping followed by thermal desorption. For higher energies direct inelastic scattering is observed, and the scattering channel is favored by a high initial energy, a large incident angle and a high surface temperature. Results from simulations are found to agree well with experimental data, although the simulations overestimate the energy transfer by approximately 10%. The results confirm that Ar collisions with ice surfaces are highly inelastic and characterized by very effective transfer of energy to surface modes. This indicates that molecules with a mass similar to Ar will trap on the surface of water ice particles with a high probability under stratospheric conditions.