The specular intensity of nearly mono-energetic helium atom beams (Ei = =(7+ZO)meV) has been measured during epitaxial deposition of Pb films on Pb(ll1) and Cu(ll1) surfaces. For Pb on Pb(ll1) regular monotonically damped oscillations are observed indicating monolayer by monolayer growth. For Pb on Cu(ll1) a more complicated modulated structure was found indicating an alternation between single-and double-layer growth mechanisms where in the double-layer regions the reflectivity was largest. The thickness at which double-layer growth predominates is found to coincide with predictions involving the dimensions of potential wells and electronic states normal to the surface. The lack of complex structures for Pb on Pb(ll1) system confirms the requirement of an internal boundary and appears to rule out other possible explanations.
Metastable and stable phases of water adsorbed at low coverages on Cu(111) were studied using vibrational spectroscopy (both time-resolved and high-resolution electron energy-loss spectroscopies) and temperature programmed desorption. The two water species have distinct spectroscopic signatures and different desorption kinetics. The more stable species appears to be three-dimensional clusters of bulklike, polycrystalline ice while the metastable species was observed to have a higher desorption rate despite a larger activation enthalpy for desorption. A simple model is proposed to explain this intriguing result. The conversion from the metastable to the stable species, as well as the affects of preadsorbed carbon monoxide on the adsorption/desorption kinetics, are also discussed.
The dissociative adsorption of cyclopentadiene (C(5)H(6)) on Cu(111) yields a cyclopentadienyl (Cp) species with strongly anionic characteristics. The Cp potential energy surface and frictional coupling to the substrate are determined from measurements of dynamics of the molecule together with density functional calculations. The molecule is shown to occupy degenerate threefold adsorption sites and molecular motion is characterized by a low diffusional energy barrier of 40±3 meV with strong frictional dissipation. Repulsive dipole-dipole interactions are not detected despite charge transfer from substrate to adsorbate.
Two-dimensional self-diffusion processes at surfaces can be studied on an atomic scale with quasielastic scattering of low-energy He atoms. The analytical strength of this new application of Heatom scattering is demonstrated for the Pb(110) surface at temperatures close to the bulk melting point, T =600.7 K. The width of the quasielastic-scattering energy distribution of diffusely scattered He atoms is a direct measure of the lateral atomic mobilities at the surface. The results show that at T)-'T the atoms of this surface have noticeable lateral diffusivities. Above-535 K the surface rnobilities exceed the bulk-liquid value. Measurements of the quasielastic energy broadening as a function of parallel momentum transfer provide direct information on the surface diffusion mechanisms. The results exhibit a strong directional anisotropy. The diffusion can be described in terms of jumps along the [110] and [001] directions. Jump lengths along the close-packed [110] 0 direction seem to be continuously distributed around an average jump length of-4.4 A. Along [001]the diffusion proceeds in jumps over single lattice spacings.
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