MULTIPHONON SCATTERING OF THERMAL-ENERGY HE ATOMS FROM THE LIF(001) SURFACE

Results from die scattering of nearly monoenergetic (DELTAE/E almost-e qual-to 2%) thermal energy He atoms from the LiF(001) surface are pres ented and discussed with particular emphasis on multiphonon interactio ns. These experiments represent the next step in a series of experimen ts exploring the multiphonon scattering of He from alkali halides in t hat previous work [G. G. Bishop et al., Phys. Rev. B 47, 3966 (1993)] has now been extended to include nonspecular angles. The LiF surface w as prepared as in the previous experiments by cleaving the crystal in air and cleaning it by sublimation in the ultrahigh vacuum system. The measurements consist of time-of-flight spectra of the scattered He at oms taken for a number of incident angles as a function of the surface temperature ranging from 290 to 720 K and for He incident wave vector s ranging from 7 to 11 angstrom-1. Analysis of the data was carried ou t using a theoretical model developed by Manson [Phys. Rev. B 43, 6924 (1991)], which allows the determination of the interaction potential between the incident atoms and the LiF surface by isolating die scatte ring form factor. The good representation of die multiphonon scatterin g found from die model allows this background to be subtracted from th e time-of-flight spectra to enhance the single-phonon He atom scatteri ng signal, which in turn permits improved surface phonon dispersion me asurements and interpretation.