High-resolution helium-atom-scattering measurements of angular distributions and time-of-flight spectra are reported for W(001) in the temperature range from 200 to 1900 K. Angular variations in the total intensity show a reconstruction related superstructure peak whose angular position and intensity are temperature dependent in the range from 400 to 220 K. At 220 K the peak intensity is maximum and its position corresponds to that expected for a reconstructed (&2X&2) 845' low-temperature phase. Time-of-flight spectra show that the behavior of the superstructure peak at higher temperatures is attributable to the elastic component and must therefore be due to a surface structural feature. Of the measured phonon inelastic peaks in the time-of-flight spectra, the one assigned to a longitudinal mode shows the typical strong temperature dependence of a soft mode in the temperature range 450~T~220 K. For T~220 K two new phonon modes appear, associated with the Brillouin-zone boundary corresponding to the c(2X2) phase. Both observations suggest that the overall phase transition on W(001) is of the "displacive" and not the "order-disorder" type, as inferred from several recent structural studies.All experimental findings are consistent with a charge-density-wave mechanism driving the reconstruction in agreement with the recent theory of Wang and Weber [Phys. Rev. Lett. 58, 1452.

Collective vibrations of the clean and hydrogen-saturated W(l 10) surface have been investigated with high-resolution inelastic helium-atom scattering. In contrast to the clean W(l 10) surface which exhibits "normal" surface phonon dispersion, the hydrogen-saturated surface shows a pronounced surface phonon anomaly in terms of a sharply defined dip in the surface phonon dispersion at Q *0.9 A "' along the F/7 ([001]) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.PACS numbers: 79.20. Rf, Hydrogen adsorption on metal surfaces leads to a great variety of surface structures and adsorbate-induced reconstructions [1], and powerful theoretical techniques have been developed to study the dynamics of these hydrogen phases [2]. As a common rule, chemisorbed hydrogen renders densely packed metal surfaces chemically inert and dynamically stable at saturation coverage. Such a behavior indicates that all broken bonds become saturated upon hydrogen adsorption. In this Letter we report on a spectacular exception to this rule: For the first time a strong surface phonon anomaly has been detected which is brought about by saturating a metal surface with chemisorbed hydrogen. The anomaly consists of a sharply defined soft elementary excitation at an incommensurate wave vector. Extensive studies of clean and hydrogen-covered W(110) surfaces have previously been undertaken [3-6]. In particular, the phase diagram with respect to H adsorption is well known [7]. From electron-energy-loss spectroscopy measurements [8] it was concluded that the hydrogen atom is bonded in the long bridge site above the hourglass-shaped hole between the tungsten atoms in the surface layer. In recent LEED studies [9] a loss of symmetry in the diffraction pattern was observed for hydrogen coverages exceeding half a monolayer. This asymmetry has been interpreted by Chung, Ying, and Estrup[9] as being due to an adsorbate-induced shift of all tungsten atoms in the topmost layer along the (001) direction. Even though such a reconstruction leads to a (lxl) superstructure it changes the environment of the tungsten atoms in the surface and should bear on the surface lattice dynamics.The present measurements were performed in a helium scattering apparatus [10] suited for elastic and inelastic (time-of-flight) measurements.The 5 x 10 x 1 -mm 3 tungsten crystal specimen was prepared using standard techniques [11], kept in ultrahigh vacuum, and characterized with LEED and Auger electron spectroscopy (AES). After cleaning, the carbon AES signal was below the detection limit of the analyzer.Elastic helium scattering from the cleanW(HO) surface along the [001], the [HO], and the [ill] directions in the surface yields sharp reflections with weak diffracted intensities typical for weakly corrugated and 2846

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The phonon dynamics of the Sn/Ge(111) interface is studied using high-resolution helium atom scattering and first-principles calculations. At room temperature we observe a phonon softening at the Kmacr; point in the (sqrt[3]xsqrt[3])R30 degrees phase, associated with the stabilization of a (3x3) phase at low temperature. That phonon band is split into three branches in the (3x3) phase. We analyze the character of these phonons and find out that the low- and room-temperature modes are connected via a chaotic motion of the Sn atoms. The system is shown to present an order-disorder transition.