Inelastic molecular beam scattering from solid surfaces

Abstract

Recent experiments have demonstrated the effectiveness of helium atomic beam scattering in characterizing the surface phonon spectrum of alkali halide single-crystal surfaces. Surface phonon dispersion curves have been measured for LiF, NaF, and KC1 (001) surfaces along the 〈100〉 azimuth and for LiF(001) along the 〈110〉 azimuth. While the results for NaF and KCl are substantially in agreement with theoretical predictions, the Rayleigh dispersion curve for LiF(001) lies considerably lower in frequency at the zone boundary than expected, possibly reflecting effects associated with surface relaxation. Beyond allowing the surface phonon dispersion curves to be measured, the experimental time-of-flight spectra and angular distributions of scattered intensity offer considerable insight into the helium-surface interaction. In particular, it is observed that incident beam parameters may be established for which single-phonon scattering dominates over multi-phonon interactions. The impinging helium atom is seen to interact with both discrete surface modes and with the “bulk” bands at the surface, the relative intensities of the two interactions agreeing well with theoretical predictions.