Phonon inelastic scattering of He atoms from single crystal surfaces

The inelastic scattering of He atoms provides a direct method for studying the surface phonon dispersion curves of single crystal surfaces. A highly monochromatic intense beam of atoms with a velocity spread Δv/v≲1% at typically 20 meV kinetic energy is scattered from the clean surface prepared and analyzed in an UHV chamber. The time-of-flight distributions reveal up to six sharp maxima which can be attributed to the annihilation or creation of single surface phonons. From the measured energy loss and the scattering angles the frequency and wave number of the interacting surface phonons are determined. Using this technique the Rayleigh mode dispersion curves of LiF, NaF, NaCl, and KCl have been measured out to the zone boundary in recent years. Here we report on new measurements at higher energies (100 meV) which reveal the optical mode S2 and the Lucas mode S4 in NaF. We also present the first measurements on a semiconductor (GaSe), which also yield dispersion curves for an optical mode as well as for the Rayleigh mode. Finally, recent measurements for metal surfaces will be described. For Ag(111) the Rayleigh mode agrees with theory but another observed dispersion curve, attributed to a longitudinally polarized acoustical mode, lies 35% lower than predicted by theory. Some preliminary results for Ni(100) are reported and compared with recent EELS measurements.