Combined He-atom scattering and theoretical study of the low-energy vibrations of physisorbed monolayers of Xe on Cu(111) and Cu(001)

Abstract

The surface phonon dispersion curves of commensurate Xe monolayers on Cu(111) and incommensurate Xe monolayers on Cu (001) surfaces have been measured using He atom scattering (HAS) time of flight (TOF) spectroscopy. The TOF spectra are interpreted by combining quantum scattering calculations with the dynamical matrix description of the surface vibrations. Both a vertically polarized Einstein-like mode and another, acoustic-like mode of dominantly longitudinal character, are identified. The latter mode is characterized by the presence and absence of the zone center frequency gap in the commensurate and incommensurate adlayers, respectively. The microscopic description of the TOF spectral intensities is based on the extensive theoretical studies of the interplay of the phonon dynamics, projectile-surface potentials, multi-quantum interference and projectile recoil, and their effect on the HAS spectra. Both single and multi-quantum spectral features observed over a wide range of He atom incident energies and substrate temperatures are successfully explained by the theory.

Diffraction of He atoms from Xe monolayer adsorbed on the graphite (0 0 0 1) revisited: the importance of multiple scattering processes

“…Xe/Cu(100) and Xe/Cu(111), have been presented in Ref. 9. There, it was found first experimentally, and then confirmed theoretically that also in these systems the adlayer S-mode is to an excellent approximation completely dispersionless.…”

Excitation of the shear horizontal mode in a monolayer by inelastic helium atom scattering

“…͑17͒ for two xenon cases, Q = 0.25 Å −1 at 2.6°from ⌫M and Q = 0.125 Å −1 at 0.5°from ⌫M. Even for the LA and S excitation probabilities, which are nonzero already in the DWBA calculations, 5 there were major changes in the intensities in the first Brillouin zone, g ជ = 0. With the g ជ 2 0 terms removed, the LA intensities were, respectively, a factor of 0.2 and 1.2 times the result of the full calculation and the S intensity was reduced by about 60%.…”

“…The question arises how much of the physics might be recovered with a more approximate theory such as distorted wave Born approximation ͑DWBA͒. 35 As presently implemented, 5,7,21 the distorted wave is the solution in the laterally averaged potential V 0 , Eq. ͑7͒, and the absence of SH excitations in the calculations then follows quite directly from a vanishing scalar product Q ជ · ⑀͑Q ជ ,SH͒.…”

“…It was expected, 5 on the basis of quite general symmetry arguments, that there should be no excitation probability for the SH modes at wave vectors in the first Brillouin zone when the scattering plane is oriented along the high symmetry monolayer axes ⌫M and ⌫K, i.e., for ␣ = 0. This is also the result in our calculations.…”

“…Some HAS experiments on xenon monolayers were interpreted 3,5 as showing that the interaction energy between two monolayer atoms is greatly changed from that for an isolated xenon pair. Nevertheless, if the polarization selection rule is ignored, observed spectra for monolayers of xenon, krypton, or argon can be understood in terms of unperturbed adatom-adatom forces.…”