Measurement of Surface Phonon Dispersion Curves of Alkali Halide Single Crystals by Time of Flight Spectroscopy of He Atom Beams

Abstract

The development of highly expanded He nozzle beams with velocity half widths less than 1% has made it possible to observe the annihilation and creation of single surface phonons in time of flight spectra of the scattered atoms. Measurements have been carried out for a wide range of incident angles at a beam energy of about 20 meV for the following crystals and directions: LiF ⟨100⟩, LiF ⟨110⟩, NaF ⟨100⟩, KCl ⟨100⟩, NaCl ⟨100⟩, and MgO ⟨100⟩. In all cases the spectra show up to 5 sharp well resolved maxima. From the final angle and energy of each peak the parallel surface momentum component and frequency of the phonon is determined. The results show a predominant coupling with Rayleigh surface phonons with only a weak coupling with bulk phonons. Rayleigh dispersion curves have been measured out to the zone boundary for all crystals except LiF ⟨110⟩ and MgO. There is also some evidence for excitation of modes of higher frequencies. Except for LiF ⟨100⟩ at the zone boundary the results compare well with theoretical calculations. Benedek has been able to fit the new LiF He atom and previous neutron dispersion curves with a revised breathing shell model calculation.