L. Mattera

Selective adsorption of 1H2 and 2H2 on the (0001) graphite surface

Abstract Scattering of 1 H 2 and 2 H 2 from a (0001) graphite surface at 100 K has been performed to study the laterally averaged molecule-surface interaction potential. The bound state spectrum has been determined by selective adsorption measurements. Energy levels at 1.4 6 , 3.6 1 , 7.9 6 , 15.3 3 , 26.4 3 , 41.6 1 meV for 1 H 2 and at 1.9 3 , 3.7 8 , 6.3 7 , 10.0, 15.4, 23.1 meV for 2 H 2 were detected with an experimental accuracy of 0.25 meV. The two lowest levels for 2 H 2 could not be observed in our experimental conditions. The experimental energy levels are in excellent agreement with those calculated for the potential V(z) = D[(1 + λ z p ) −2p − 2(1 + λ z p ) −p ] with D = 51.7 ± 0.5 meV , p = 3.9 ± 0.2 and λ = 1.45 ± 0.02 A −1 . A comparison with the theoretical model proposed by Crowell [1] to describe the interaction of non polar molecules with graphite has been carried out. Improvements to the Crowell model taking into account either higher order dispersion terms or the exponential behaviour of the repulsive interaction, are discussed.

Deep ultraviolet plasmon resonance in aluminum nanoparticle arrays.

Small aluminum nanoparticles have the potential to exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum, however technical and scientific challenges make it difficult to attain this limit. We report the fabrication of arrays of Al/Al2O3 core/shell nanoparticles with a metallic-core diameter between 12 and 25 nm that display sharp plasmonic resonances at very high energies, up to 5.8 eV (down to λ = 215 nm). The arrays were fabricated by means of a straightforward self-organization approach. The experimental spectra were compared with theoretical calculations that allow the correlation of each feature to the corresponding plasmon modes.

Vibrational spectrum of H and D on the (0001) graphite surface from scattering experiments

We have investigated the vibrational spectrum of hydrogen and deuterium atoms adsorbed on the (0001) surface of a graphite single crystal by means of atomic beam scattering in UHV conditions. Bound state resonances, which appear as minima in the specularly reflected beam have been detected over a wide range of incidence and azimuthal angles. Energy levels at 31.6±0.2 and 15.3±0.3 meV for 1H and at 35.4 ±2; 21.35±0.1; 12.0±0.1, and 5.9±0.3 meV for 2H have been determined. By fitting the experimental energy levels to those calculated with the potential the following values of the parameters are obtained: D=43.3±0.5 meV; λ=1.36±0.03 A−1; p=4.4±0.6. The hypothesis that the interaction potential is the sum over all crystal layers of an atom–single layer potential V*(z) is used. V*(z) has a well depth D*=39.2±0.5 meV and a long range behavior given by C*4/z4 with C*4=5.2±0.3 eV A4.

XPS analysis of the surface composition of niobium for superconducting RF cavities

Abstract The surface chemical composition of Nb samples prepared by following procedures commonly employed in the preparation of RF cavities has been studied by XPS and ARXPS. In order to understand the process occurring at surface, the thermal evolution of the surface has been studied in the 30°C÷1000°C range of temperature both in the heating and cooling phases. During the heating phase, an irreversible transition has been observed near T ≅200÷300°C; it is characterized by a progressive reduction of Nb oxides from Nb 2 O 5 to NbO 2 and finally to NbO. The O signal disappears near T ≅1000°C and it reappears below 900°C, during the cooling phase. This can be interpreted as due to O migration in the Nb matrix, raising T , and to O diffusion towards the surface as the temperature decreases. Work function measurements show that the values of Φ are strongly correlated with the chemical composition of the surface.

A simple and flexible atom-surface potential with easy-to-handle vibrational spectrum

Abstract A highly flexible empirical potential Vp(z) suitable to describe the main features of the laterally averaged atom-surface interaction is presented and an accurate expression for the vibrational spectrum of the atom in Vp(z) is given. This establishes a simple method of inversion of the atom-surface vibrational spectra particularly suitable for levels located in the deepest 9/10 of the potential well. The present method is complementary to the one introduced by Le Roy [5].

Surface magnetism of oxygen adsorbed on a FeAg(100) film

Abstract The influence on surface magnetism of oxygen adsorption at T = 120 K on an iron film grown on Ag(100) has been analysed by spin polarized metastable deexcitation spectroscopy. The low temperature has been chosen in order to reduce the segregation of silver at surface. SPMDS data have been interpreted by a model that provides the effective charge and magnetization densities. At 120 K, oxygen chemisorbs atomically, the O2p states are spin polarized and show a ferromagnetic coupling with the substrate. The energy shift between the majority and the minority density of states is a function of exposure as it decreases from 0.24 eV at 0.25 L to zero for exposures higher than 1.6 L. At exposures of about 1 L, oxygen starts a process that leads, at 3–4 L, to the formation of a layer that is magnetically dead as observed by He∗.

Uniaxial magnetic anisotropy tuned by nanoscale ripple formation: Ion-sculpting of Co/Cu(001) thin films

We have investigated the growth of surface nanostructures on a Co/Cu(001) film and the growth of Co films on a nanostructured Cu(001) substrate as well as the effect of nanoscale pattern formation on the film magnetic properties. Here we demonstrate by scanning tunneling microscopy measurements and magneto-optic Kerr effect hysteresis curves that low-temperature grazing-incidence ion sputtering can be used to induce the formation of nanoscale ripples which reduce the four-fold symmetry of the Co film to two-fold, thus generating a strong in-plane uniaxial magnetic anisotropy. The nanostructures and the associated uniaxial magnetic anisotropy were found to be stable up to room temperature.

Surface scienceSurface magnetism of oxygen adsorbed on a FeAg(100) film

Abstract The influence on surface magnetism of oxygen adsorption at T = 120 K on an iron film grown on Ag(100) has been analysed by spin polarized metastable deexcitation spectroscopy. The low temperature has been chosen in order to reduce the segregation of silver at surface. SPMDS data have been interpreted by a model that provides the effective charge and magnetization densities. At 120 K, oxygen chemisorbs atomically, the O2p states are spin polarized and show a ferromagnetic coupling with the substrate. The energy shift between the majority and the minority density of states is a function of exposure as it decreases from 0.24 eV at 0.25 L to zero for exposures higher than 1.6 L. At exposures of about 1 L, oxygen starts a process that leads, at 3–4 L, to the formation of a layer that is magnetically dead as observed by He∗.

Representation of potential energy curves for diatomic molecules

Power series expansion for molecular potentials is given with improved convergence properties relative to the Dunham expansion. (AIP)

Study of the Ag(110) surface by He diffraction

Abstract The scattering of He beams from the Ag(110) surface has been studied for two beam energies and several angles of incidence. The analysis of the experimental data shows that the softness of the interaction potential must be carefully considered. A soft corrugated wall (SCW) potential model which allows fast and accurate calculations is presented. The force parameter and the surface corrugation, strongly dependent on the incident energy, are derived as best fit parameters and do favourably compare with the values obtained by close-coupling calculations. The peak-to-peak corrugation is found to be significantly larger than for Ni(110) and Cu(110) surfaces.