ELASTIC-SCATTERING, STICKING AND ACCOMMODATION OF NOBLE-GAS ATOMS AT A COLD SURFACE

The importance of elastic scattering (Debye-Waller factor P0) both for the sticking and for the accommodation coefficient (s, alpha) is shown. Forced oscillator calculations applied to a soft cube model are valid over a broad range of gas energies and masses. At a cold surface s and a approach 1-P0 for small energies, whereas for a classical calculation neglecting the phonon quantization they approach unity. For higher energies alpha vanishes while a approaches the Baule limit. The question whether s and a can increase with increasing surface temperature T(s) due to the decay of the elastic line cannot be definitively answered in the frame of the simple (prompt) kinetics presented in this paper. The inequality 1 greater-than-or-equal-to alpha greater-than-or-equal-to s seems to be practically fulfilled, but is strictly proved for monoenergetic beams at T(s) = 0 only.