Probing a cold surface with slow heavy-atom scattering: Experimental results and theoretical calculations

Slow heavy atoms scattering from cold surfaces excite many phonons, but still have a finite elastic scattering fraction. We have measured the coherent elastic intensity of low-energy Ne, Ar, and Kr beams scattered from a Cu(111) surface, and compared the results to accurate semiclassical calculations. Earlier experiments have been extended to a range of incident beam energies, while earlier calculations have been improved by using state-of-the-art atom-surface potentials and surface phonon densities of states. All inputs to the calculations are taken from the literature, so they involve no adjustable parameters. We compare both the zero-temperature limit and the thermal attenuation of the elastic scattering at elevated temperatures. Our comparisons confirm that the probability for elastic scattering is (i) independent of particle mass m at low surface temperatures and (ii) depends on $\sqrt{m}$ at elevated temperatures. Corresponding experimental observations for the comparatively light particles ${\mathrm{H}}_2,$ ${\mathrm{D}}_2,$ and He show clear deviations from the semiclassical predictions. Our analysis shows that this regime provides new and complementary information about both surface phonons and atom-surface potentials.