Measurement of the Rayleigh surface-phonon dispersion curve for NaCl(001) from high-resolution He time-of-flight spectroscopy and from kinematical focusing angles

Angular distributions and time-of-flight spectra have been measured for the collisions of 20-meV He atoms with a NaCl(001) surface in the 100> direction. Structures in the angular distribution and in the time-of-flight spectra due to elastic and inelastic scattering are measured and used to extract (1) a corrugation parameter of the rigid-atom surface potential ζ₀=0.34 {}, (2) three bound-state energies of the He-surface potential, and (3) a surface-phonon Rayleigh dispersion curve out to the edge of the Brillouin zone. Furthermore, we were able to demonstrate that by rotating the crystal about the surface normal it is possible to unambiguously identify kinematical focusing features in the angular distributions. These features were then used to determine a surface-phonon dispersion curve in good agreement with that obtained from the time-of-flight experiments.