A comprehensive experimental study of the dynamical interaction of He atoms with Cu(001) surface phonons

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Abstract
We present new measurements of inelastic He atom scattering from the surface phonons of Cu(001) as a function of crystal temperature, incident energy, and parallel momentum transfer. A careful subtraction of the multiphonon intensity and other background contributions from the time-of-flight intensities reveals three distinct surface-localized vibrational modes which are ascribed to the Rayleigh phonon, the longitudinal bulk resonance, and a further acoustic bulk resonance at higher energy transfers. The longitudinal resonance couples very strongly to the scattering He atoms and, for a wide range of incident conditions, gives peaks which are more intense than those due to the Rayleigh mode. The energy and momentum dependence of these peak intensities are analyzed with the aid of a simple distorted wave Born approximation, and the different coupling parameters for the two modes are determined and compared with other available data. The incoherent diffuse elastic peak is shown to decrease as a function of parallel momentum transfer according to the theory of Fraunhofer scattering from a random array of point defects. The multiphonon background is shown to be in agreement with a quick scattering approximation.