Origin of the anomalous low-frequency losses observed in the inelastic scattering of He atoms from LiF(001)

The anomalous-loss features below the Rayleigh surface-phonon dispersion curve, which have recently been reported by Brusdeylins et al. in their time-of-flight measurements of He atoms inelastically scattered from LiF(001), are shown to be a consequence of surface diffraction arising from a spread in particle velocities within the He beam. That is, the phenomenon is an extrinsic diffraction effect rather than an inelastic process associated with an intrinsic physical property of the crystal surface