Helium atom scattering: a gentle and uniquely sensitive probe of surface structure and dynamics

The scattering of molecular beams of 4 He atoms from solid surface provides a very sensitive diffraction probe of surface structure and surface dynamics. The experimental method is described and illustrated by some studies of surface structure of metal surface. Because of the high monochromaticity of the helium beams ( Delta nu / nu approximately=1%) surface phonons with energies from 0.5 meV upwards can be resolved and entire surface phonon dispersion curves can be measured. On Cu(111) and Cu(001) an anomalous longitudinal resonance with large intensity is observed. On W(001) and W(110)+H(1*1) sharp anomalies give evidence of electron-phonon coupling which in the former system is related to the reconstruction at 280 K. Finally, two experiments on the dynamics of thin films as a function of layer thickness are described.