A study of the (001)LiF surface at 80 K by means of diffractive scattering of He and Ne atoms at thermal energies

Abstract

Molecular beam techniques were employed to investigate the scattering of nearly monoenergetic He and Ne atoms of thermal energies from the clean (001) face of LiF at low temperature. By using good angular resolution and sensitivity, diffraction peaks were resolved over a wide range of incident and final angles. Taking into account the velocity distribution within the supersonic beam, the diffraction probabilities of the elastic peaks are extracted from the measured scattered intensity. A comparison with elastic diffraction theories is given, particularly with the quantum theory of surface rainbow developed by Levi and coworkers. This theory is found to be in satisfactory agreement with the experimental results. Information on the surface roughness is obtained: to the exploring incident atoms the surface appears as a simple sinusoidal hard wall in two dimensions, with a corrugation amplitude of about 0.3 Å. Qualitative information on the attractive part of the atom-surface potential is also obtained. Owing to the low surface temperature, the inelastic scattering is relatively weak. From the measurement (for helium), or from the evaluation (for neon) of the total elastic scattered intensity observed at normal incidence, a Debye-Waller factor of 0.25 for He, and of 0.037 for Ne at 80 K is derived. The surprisingly large value found for NeLiF cannot be explained by the conventional theory, and is suggested to be connected with the long collision time. The inelastic structures observed in the scattering of neon, particularly at large outgoing angles, are discussed.