Observation of Surface Phonons in Inelastic Scattering of He Atoms from LiF(001) Crystal Surfaces

Preliminary results are reported on the inelastic scattering of low-energy (20 meV), monoenergetic ($\frac{\Delta v}{v}=0.8\%$ full width at half maximum) He nozzle beams from a LiF(001) surface along the $〈100〉$ direction. The time-of-flight spectra reveal both broad and extremely sharp maxima. The broad maxima are attributed to a distribution of phonons of bulklike character with small momenta, while the sharp maxima indicate excitation of single discrete surface modes.