Robert L. Palmer

Helium scattering as a probe of the clean and adsorbate covered Pt(111) surface

Abstract Thermal energy (16–165 meV) helium nozzle beam scattering has been used as a probe of the physical structure of the nominal Pt(111) surface and for the consequential effects of the adsorption of gases including CO, H 2 , O 2 and C 2 H 2 . By careful crystal alignment and polishing techniques a Pt(111) surface with terrace widths greater than 3000 A was achieved as judged by the helium scattering. The profile of the scattered peak was immeasurably affected by the presence of various adsorbates while the intensity decreases monotonically with coverage. The sensitivity of the scattered intensity to adsorbates is, in general, a function of the energy and angle of incidence of the helium beam. The scattering cross-section of the adsorbate can be, for some cases, much larger than its Van der Waals diameter and apparently increases with the degree of perfection of the surface.

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

Abstract CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ co not relate to any observed LEED superstructure. The value of Δφ min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10 −3 at a temperature of 300 K, Δφ min = −240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 A 2 , i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10 −2 and less than 10 −3 , respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.

Thermal energy helium scattering from CO/Pt(111) at very low coverages

We report on thermal He scattering from CO/Pt(111) at low coverages (θCO<0.01), which was shown to be dominated by long-range attractive forces centered at the adsorbed CO molecules. The scattering cross section of adsorbed CO is measured as a function of both the velocity of the incident He atoms and the angle of incidence. In first approximation the angular dependencies can be understood in terms of simple geometric “shadowing like” effects. The velocity dependencies, measured at different angles of incidence, coincide on a universal curve after correction for refractive and geometric effects.

Symmetry breaking commensurate-incommensurate transition of monolayer Xe physisorbed on Pt(111)

Abstract We report a high resolution He-diffraction study of the commensurate-incommensurate transition of monolayer xenon physisorbed on Pt(111). The experimental results show that we have been able to observe for the first time a (√3x√3)R300 commensurate (C) striped incommensurate (SI) transition. The striped domain walls are found to run into the ΓK -direction, i.e. the uniaxial compression is in the ΓM -direction. The C-SI transition appears to be continuous within the experimental accuracy and the incommensurability in the weakly incommensurate phase follows a 1 2 power law versus reduced temperature.

Thermodynamic measurements of Xe-adsorption on Pt(111)

Abstract The adsorption of xenon on a nearly defect free Pt(111) surface has been studied by elastic and inelastic He-scattering. With decreasing temperature the sequential of 2D-gas, monolayer, bilayer and multilayer films are observed. The thermodynamic phase diagram and the latent heats of adsorption are determined.

Surface phonon dispersion of clean and oxygen covered Pt(111)

Abstract The dispersion of the vibrational modes of the Pt(111) surface in the absence of impurities as well as in the presence of oxygen has been measured by inelastic helium scattering along the \ gG

The Novaco-McTague rotated Xe monolayer on Pt(111): A high-order commensurate locked phase

M azimuth. In the case of the ordered p(2×2)O superstructure, the additional dispersion branch appearing as a result of the Brillouin zone folding was also measured. The experimental results show the first direct evidence for the appearance of a surface Rayleigh phonon gap at the new zone boundary. The gap was determined to be ~ 0.85 meV at the M 0 point. Lattice-dynamical calculations show that this relatively large gap is due to bond stretching interactions between the adsorbate and substrate atoms. The dispersion curve in the presence of disordered oxygen, obtained by annealing (1200 K) the Pt surface in an oxygen atmosphere and assumed to be located subsurface on the basis of ion scattering data has been measured. The frequency of the Rayleigh phonon at the M point is 12% below the clean surface value. This is at variance with the case of disordered oxygen on metal surfaces (O/Ni(100)) which has been shown to have a negligible influence on the Rayleigh dispersion curve. According to the lattice dynamical calculations this 12% frequency lowering corresponds to a reduction of the interlayer force constant k 12 by about 30% and thus supports the subsurface location of the oxygen atoms.

A thermal He scattering study of CO adsorption on Pt(111)

The structural analysis of the physisorption system Xe/Pt(111) by means of high-resolution helium diffraction shows that the Xe monolayer exhibits a variety of structural phases: commensurate, incommensurate and “incommensurate” rotated. The observed buckling of the rotated phase demonstrates that a fraction of the Xe atoms is locked in high symmetry sites in agreement with the theoretical “coincident site lattice” concept of Fuselier et al., i.e. the rotated phase is a higher-order commensurate phase.