Using inelastic scattering of thermal energy He atoms we have determined the phonon dispersion curves in commensurate ͑ p 3 3p 3 ͒R30 ± Xe monolayers adsorbed on Cu(111). In addition to the well known transverse acoustic mode the dispersion curve of the longitudinally polarized phonon branch could be measured. Since this is the first case where a finite zone-center phonon gap (here 0.4 6 0.15 meV) has been seen for a Q-resolved phonon branch, a normal mode analysis and theoretical calculations on the phonon excitation probabilities have been carried out to confirm the assignment. From the gap magnitude important information on the corrugation of the Xe-atom-substrate potential is derived. [S0031-9007(97)04932-6] PACS numbers: 68.35.Ja, 63.22. + mRecently the properties of monolayers of noble gas atoms, in particular Xe, adsorbed on solid substrates have attracted interest in connection with fundamental experiments on the microscopic origin of friction carried out by Krim and co-workers [1]. Up to now, however, no consensus has been achieved concerning the proper theoretical modeling of these layers. Two recent attempts to theoretically analyze the results of the friction experiments [1] using molecular dynamics have obtained rather different results [2,3]. These discrepancies may in part be due to uncertainties concerning the proper model potentials describing the corrugation of the metal substrate as seen by the Xe atoms. These uncertainties result from the lack of direct experimental information on the potential energy surface governing the lateral motion of Xe atoms on the substrate. So far it has only been possible to extract indirect information from, e.g., detailed analyses of thermodynamical properties and thermal desorption spectroscopy [4].In the present Letter we will describe the outcome of experiments where high resolution scattering of thermal energy He atoms (HAS) has been successfully used to determine the dispersion of the longitudinal phonon in the commensurate ͑ p 3 3p 3 ͒R30 ± Xe overlayer on Cu(111). A detailed analysis of this mode provides two important pieces of information: (1) At the zone center of the surface Brillouin zone [cf. Fig. 1(a)] this mode corresponds to a lateral displacement of the whole Xe layer, and the corresponding frequency thus directly measures the curvature of the potential energy surface describing the interaction of the individual Xe atoms with the substrate.(2) From the slope of the longitudinal phonon dispersion curve (or the velocity of sound) the force-constant coupling between adjacent Xe atoms can be obtained. As to (1) a Q-resolved phonon branch with a finite zone-center phonon gap has not yet been seen experimentally, although in previous neutron scattering studies this quantity has been extracted from the total density of states [5]. With regard to (2) one may expect the force-constant coupling of the adsorbed Xe atoms to be very similar to that predicted from accurate gas-phase potentials [6] or from the analysis of bulk phonon data [7], but recent work ha...

The structure and phonon dynamics of the ice surface, prepared by growing thick ice layers (ϳ100 Å) on Pt(111), were investigated using high resolution helium atom scattering. Ice layers grown at T s 125 K were found to be well ordered, with a complete bilayer (1 3 1) surface termination, as shown by diffraction measurements at T s 30 K. Time-of-flight spectra provide evidence for an intense longitudinal shearing mode, large multiphonon background, and enhanced vibrational amplitudes at the surface, which are consistent with dynamic disorder and a large accommodation coefficient at the surface. [S0031-9007(98)

Thin films of lead have been grown on the Cu(111) surface and their structure and phonon dispersion curves have been investigated with high-resolution helium atom scattering. The diffraction pattern of the first monolayer (ML) indicates a p(4×4) structure. Films consisting of N=3, 4, 5, and bulklike 50 ML all have a regular (1×1) structure with the same lattice constant as the bulk and small superimposed p(4×4) peaks for the thinnest films. The time-of-flight spectra reveal an unusually large number of inelastic peaks for all the films. The results have been analyzed in terms of dispersion curves which exhibit more than 1/2 the total number of expected two N modes active in the planar scattering geometry. Dispersion curves for an unannealed 5 ML film are also reported. A force-constant model fitted to the bulk dispersion curves can only qualitatively reproduce some of the data but help to explain some features. Along the Gamma −Kappa direction, a Kohn anomaly is identified for the 3, 4, and 5 ML films at nearly the same wave vector as in the bulk along the equivalent direction

The damping of molecular motion relative to a solid substrate was investigated by high-resolution vibrational spectroscopy in the far-infrared regime. For octane adsorbed on a hydrogen-passivated Ru(0001) surface the experimental linewidths observed by He-atom scattering for the frustrated translation normal to the surface are in accord with a damping by phonon emission, but a strong broadening is found for the clean Ru substrate. This enhanced friction is related to an increase in the rate of electron-hole ͑e-h͒ pair creation due to the presence of hybrid metal-molecule electronic states which are absent for the hydrogen passivated surface. Data from near-edge x-ray absorption spectroscopy provide direct evidence for these differences in electronic structure. Results from IR spectroscopy strongly suggest that the soft C-H bands observed for these systems are also related to the formation of these hybrid electronic states. [S0031-9007(97)04947-8] PACS numbers: 68.35.Ja, 33.20.Tp, 73.20.Hb An important topic in gas-surface interaction is the nature of the energy exchange between an impinging gas particle and the substrate. Whereas on insulators the energy transfer will comprise only excitation and deexcitation of substrate phonons and/or molecular vibrations, on conductors an additional energy dissipation via the excitation of electron-hole ͑e-h͒ pairs is possible. So far, attempts to directly identify the excitation of e-h pairs in high-resolution experiments for light molecules scattered off solid surfaces have failed, in accord with theoretical calculations. In case of He atoms with an energy of 22.6 meV impinging on a Cu surface at 16 K, theoretical calculations yield an integrated probability for electron-hole pair creation of 10 25 [1], which is about 4 orders of magnitude smaller than the experimentally determined total inelastic cross section. The recent experimental [2] and theoretical [3,4] success in advancing the understanding of the microscopic origins of friction has created substantial interest in a related topic, namely, that on the mechanism underlying the damping of molecular motion relative to a solid substrate. For some lubricant molecules it has been proposed that the electronic contributions to friction become so large that the e-h pair creation dominates damping by phonon emission (e.g., octane͞Cu (001) [5]).In the present study the damping of molecular motion was measured by high-resolution spectroscopy of the frustrated translation normal to the surface. This F T z mode is a particular external vibration and corresponds to a translation of the whole molecule. The dissipation of translational energy into the substrate manifests itself in a decrease of vibrational lifetime. In order to precisely monitor the line shape and the frequency of these lowenergy vibrations with frequencies in the far-infrared regime (typically below 100 cm 21 , or 12.4 meV), we have employed high-resolution He-atom scattering (HAS) to study the low-energy vibrations of octane ͑C 8 H 18 ͒, a prototype lubricant mo...

The lifetime of the Ar13+ 1s(2)2s(2)2p2p0(3/2) metastable level was determined at the Heidelberg Electron Beam Ion Trap to be 9.573(4)(5). The accuracy level of one per thousand makes this measurement sensitive to quantum electrodynamic effects like the electron anomalous magnetic moment (EAMM) and to relativistic electron-electron correlation effects like the frequency-dependent Breit interaction. Theoretical predictions, adjusted for the EAMM, cluster about a lifetime that is approximately shorter than our experimental result.