U. Valbusa

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.

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.

High-resolution electron energy-loss spectroscopy analysis of Ag(001): discovery of a new surface longitudinal mode using first-principles phonon calculations

Abstract We present analysis of electron-energy loss spectra of Ag(001) using first-principles surface phonon and multiple scattering inelastic cross-section calculations. Excellent agreement with experiment is obtained. The study reveals a new longitudinal surface phonon at M which causes the measured loss peak to shift as the incident energy is varied. These results demonstrate the importance of using accurate theory for proper interpretation of EELS spectra and indicate that force-constant models with adjustable parameters may be misleading.

Plasmon damping and surface interband transitions on Ag(001) and (011)

The dependence of surface-plasmon damping versus plasmon momentum and energy has been measured for the Ag(011) surface with angle-resolved electron energy loss spectroscopy. The data are compared with previous results obtained for Ag(001). At small transferred momenta, q‖, we observe a nearly linear dependence of the energy loss width with q‖ due to the growing phase space available for e-h pair creation. However the very efficient damping mechanism observed for q‖ beyond 0.10 A−1 is not present for Ag(011). We demonstrate that it is due the onset of interband transitions between filled and empty surface states. Two such states are indeed present near the X point of Ag(001) as demonstrated by inverse photoemission experiments and electronic band structure calculations. The energy gap between the two bands can thus be determined by EELS with a greater precision than by photoemission.

Adsorption of molecular oxygen on Ag(110)

Abstract The investigation of the initial sticking coefficent S 0 for non-dissociative adsorption of oxygen on Ag(110) along 〈 1 1 0 〉 at low temperature (83 K) is presented. The data were recorded as a function of the incident energy and of the impact angle of the molecules. S 0 is found to vary from 10 −3 to 1 in the normal energy range from 20 meV to 600 meV. The barrier is the same as with the crystal at room temperature. An evaluation of the electric dipole moment and of the charge transfer from the substrate to the adsorbed molecule is given.

Collisionally aligned molecular beams: a tool for stereodynamical studies in the gas phase and at surfaces

Production, characterization and control of alignment degree of molecules are of importance for investigating in detail the stereodynamics of elementary processes involving elastic, inelastic and reactive events and also to prepare gas-phase species for selective surface scattering investigations. The focus here is on collisional alignment in supersonic seeded molecular beams, a technique which shows perspectives on the applications, offering appealing features for ‘duty cycle’ and intensity characteristics. Attention will be addressed to recent stereodynamical studies carried out on hydrocarbon molecules in the gas phase and on applications of such aligned beams to surface scattering studies.

Quasi-elastic scattering of neon from (001)LiF surface

Abstract In scattering measurements of atoms and molecules from surfaces the intensities of the diffraction and specular peaks contain contributions from quasi-elastic scattering events, in addition to the coherent elastic scattering. These may occur because of inelastic collisions with very small changes in energy and momentum and because of elastic collisions with random defects of the surface. In the present article it is shown how to separate the quasi-elastic contribution from the elastic one by measuring both angular and energy distributions of the scattered particles. As an example the procedure is used to study the scattering of Ne from LiF in order to measure the Debye-Waller Factor (DWF) at low surface temperatures. The analysis of the DWF temperature dependence yields detailed information on the nature of the interaction, and shows good agreement with the Levi-Suhl model for the DWF.

EELS cross-section of surface phonons on Ag(001)

SummaryWe present EELS cross-section data of surface phonons on Ag(001) alongn

Comment on "Surface-plasmon energy and dispersion on Ag single crystals"

bar Gamma - bar {rm M}