Observation of selective desorption of one-phonon inelastically scattered He atoms from a LiF crystal surface

Abstract

Measurements of angular distributions for scattering of low energy (≊20 meV) monoenergetic (Δv/v = 0.8% FWHM) He beams from a LiF (001) crystal along the 〈100〉 and 〈110〉 directions and from a NaF (001) crystal along the 〈100〉 direction reveal a large number of mostly small maxima and minima between the specular and diffraction peaks. Typical intensities of these features are about 10−3 of the specular peak. Time‐of‐flight spectra of the scattered atoms at the incident angles corresponding to the maxima reveal that the atoms were inelastically scattered by single phonons. From the known bound state energies for both systems He–LiF and He–NaF, we find that most of the maxima can be explained by a two step process in which the atoms are first elastically selectively adsorbed by diffraction into an in‐plane bound state and then are subsequently desorbed as a result of a single phonon inelastic process. From the observed half‐widths the lifetimes of atoms in the trapped states are found to depend on the vibrational quantum number and for the weakest bound state is as large as 60×10−12 sec. Evidence for minima involving selective adsorption into out‐of‐plane states is also presented.