Hydrogen dissociation on high-temperature tungsten

H₂ dissociation on polycrystalline tungsten is measured from 1700 to 3000 K using the filament temperature (T) change and a normalized H-atom density at the chamber surface. The dissociation probability per H₂ filament collision (P_diss) saturates at 0.40 at high T and has a 2.25 ± 0.05 eV apparent activation energy when P_diss ? 1. This activation energy is consistent with prior data and models, but the H₂ pressure dependence is not. P_diss is independent of the H₂ pressure for this entire T range and the 1-85 mTorr pressure range studied, contradicting the primary model that has been used to explain H₂ dissociation on tungsten and other metals. We show that some apparently contradictory prior measurements are actually consistent with our observations and with each other, once this pressure dependence of P_diss is recognized.     

Multiple scattering investigation of the 1T-TaS2 surface termination

Multiple scattering theory based on a cluster model is used to simulate full hemispherical X-ray photoelectron diffraction measurements on a 1T-TaS₂(0001) surface. Key points to determine the surface termination are discussed. As the commonly applied single scattering simulations do not give satisfying results, a multiple scattering approach has to be used to accurately simulate the full hemispherical photoelectron diffraction patterns. Differences and similarities between calculations of Ta and S terminated surfaces are presented along with experimental results at room temperature using both, the single and the multiple scattering approaches. We find that the surface is S terminated and that the quantitative difference between the calculations for both terminations permits to show the limits of the single scattering approach for solving surface termination problems. Moreover, by generalizing the results obtained using the multiple scattering approach, we discuss the application of this method to other similar systems.     

Resonantly enhanced off-specular X-ray scattering from polymer/polymer interfaces<Superscript>⋆</Superscript>

We have used measurements of the absolute intensity of diffuse X-ray scattering to extract the interfacial tension of a buried polymer/polymer interface. Diffuse scattering was excited by an X-ray standing wave whose phase was adjusted to have a high intensity at the polymer/polymer interface and simultaneously a node at the polymer/air interface. This method permits the capillary-wave-induced roughness of the interface, and hence the interfacial tension, to be measured independently of the polymer/polymer interdiffusion.     

Electron-impact dissociation and transient properties of a stored LiH2 - beam

The fragmentation of LiH₂ ^- anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H₂ and Li^- + H₂ channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.     

DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

Stripping voltammetry of Pb and Cu using a microcantilever electrode

Microfabricated silicon microcantilevers coated with gold on one side have been used as working electrode in a three-electrode electrochemical arrangement. In addition to electrochemical current, cantilever bending has been used as a signal for monitoring electrode reactions on the cantilever surface. The microcantilever bending was measured by an optical beam deflection method as the surface potential was scanned and electrochemical reactions occurred on the surface. The microcantilever bending due to differential surface stress was used to sense Pb and Cu using cyclic voltammetry (CV) and linear sweep stripping voltammetry (LSSV).     

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4f_7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4f_7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4f_7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born-Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4f_7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.     

Growth and electronic properties of ultra-thin Ag films on Ni(1 1 1)

We studied the growth mode and electronic properties of ultra-thin silver films deposited on Ni(1 1 1) surface by means of scanning tunnelling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES). The formation of the 4d-quantum well states (QWS) was analysed within the phase accumulation model (PAM). The electronic structure of the 1 ML film is consistent with the silver layer which very weakly interacts with the supporting surface. The line-shape analysis of Ag-4d_xz,yz QWS spectrum support the notion of strong localization of these states within the silver layer. The asymmetry of the photoemission peaks implies that the decay of the photo-hole appears to be influenced by the dynamics of the electrons in the supporting surface.     

Surface reconstruction of clean bcc-Fe{1 1 0}: A quasi-hexagonal top-layer with periodic height modulation

Hexagonal-pillar shaped pure Fe single crystal whiskers with six {1 1 0} side planes were obtained by means of chemical vapor deposition. Atomically resolved scanning tunneling microscopy images obtained on the {1 1 0} surface showed a quasi-hexagonal atomic array with mesoscopic-range periodic height modulation of about 1/3 of an atomic step. This height modulation was found to be a result of an interference between the quasi-hexagonal top-layer and the sub-surface bcc-Fe{1 1 0} layer. Unit vectors of the mesoscopic-range modulation turned out to be expressed as , where and are the primitive vectors of the two-dimensional atomic array in the top-layer and those in the sub-surface layer, respectively. The two-dimensional density of atoms in the top-layer is slightly higher by 0.46% than that in the sub-surface layer.