Experimental results: Beyond single phonons

Abstract

The previous chapters have been devoted to summarizing the vast knowledge acquired about the single phonon vibrations of atoms in or near the surface and about atoms and molecules on the surface and their dispersion curves. This new knowledge was made possible largely thanks to the development of He atom scattering spectroscopy and the progress made in theoretical analysis. Single phonon inelastic scattering processes were implicit since only in this case is it possible to extract accurate phonon dispersion curves from the data. The high resolution of HAS spectroscopy, especially in its 3He-spin echo version also allows for the observation of dynamical phenomena far beyond the harmonic motion, like multiphonon processes involving low energy phonons or the diffusion of adsorbates on the surface. Moreover, the unique property of closed shell He atoms to exchange energy and momentum with the ion cores of the surface atoms via the virtual excitation of interposed electrons makes HAS also a valuable tool for investigating collective surface electron excitations in the THz domain. It therefore seems appropriate to discuss also some of these and other emerging areas of HAS research where new important insight can be expected in the future.