J.P. Gauyacq

Position and width of a negative ion state in front of a surface : formation of C-(4S) ions by electron capture

A new method for determining the position and width of negative ion states in front of a metal surface is presented. This coupled angular mode (CAM) method consists of studying the electron scattering by the superposition of an e−-atom (molecule) interaction and an e−-surface interaction. The surface interaction destroys the spherical symmetry of the e−-atom (molecule) scattering and mixes the various angular scattering modes. The e−1-atom (molecule) interaction is treated in the effective range approximation and then the present method is an extension of ERT to anisotropic environments; typically, it describes a negative ion in the field of its image charge. An application to the case of C− ion is presented. This system presents multichannel and orientation effects. Indeed, the partial widths of C− corresponding to the various orientations of C are found to differ by an order of magnitude. These results are used to discuss the experimental results of van Pinxteren et al. [Europhys. Lett. 10 (1989) 715] on C− formation by collision on a cesiated tungsten surface.

Ionization probabilities of oxygen sputtered from metal surfaces

Abstract The charge state of oxygen atoms sputtered from a metal surface is studied, using recent theoretical results on the O − ue5f8Al surface. These were obtained with the coupled angular mode method and fully take into account the multielectron, multistate and polarization effects of the O − ue5f8Al interaction. The O/O − ratio is determined for various velocities of the sputtered particles as a function of the metal surface workfunction. The O − fraction is found to vary very rapidly with the surface workfunction. A strong polarization effect is also observed, i.e. the O − survival probability strongly depends on its polarization ( M L = 0 or ± 1). The results are found in very good agreement with the experimental results of Yu (1981) on O − ions sputtered from O and alkali covered V and Mb surfaces.

Li+ neutralisation in back-scattering from alkali/Al(100) surfaces: comparison between the various alkalis

Abstract The Li+ ion neutralisation by back-scattering from an Al(100) surface partially covered by alkali adsorbates is theoretically studied, using a previously developed model representation of the Li+ ion–alkali adsorbate–Al substrate system [A.G. Borisov, G.E. Makhmetov, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 350 (1996) L205]. Three alkali adsorbates are studied: Na, K and Cs and the differences between their effects are discussed. In the back-scattering geometry, the neutralisation probability is found to be quite different for lithium bouncing from the alkali adsorbate or from the Al substrate; it is also different for the various alkali adsorbates. The present results are found to be in quantitative agreement with the recent experimental data of Weare and Yarmoff [C.B. Weare, J.A. Yarmoff, Surf. Sci. 348 (1996) 359]. These results confirm the importance of the local variation of the charge transfer process on the surface, as well as the capability of the charge transfer process to probe the local electronic potentials. A large probability for Li(2p) excited atom formation by collision on the adsorbate sites is also found in the present study, even in the limit of very low alkali coverages.

Atoms interacting with a metal surface with adsorbates: local and non-local effects on the charge transfer

Abstract A model system representing a collisional Li atom interacting in the “on top” geometry with an Al surface partly covered with Li adsorbates is studied. The cases of a unique adsorbate and of a uniform adsorbate layer are considered and compared. The energies and widths of the atom levels are much modified in the vicinity of the adsorbate. This is interpreted in terms of molecularisation of the atomic and adsorbate levels. These results also determine the relative importance of the local and non-local effects of the adsorbates on the resonant charge transfer process in atom-surface collisions.

Singlet to triplet conversion in low energy He metastable collisions with metal surfaces: conversion via He+ formation

Abstract Results of a quantitative study of the characteristics (energy position and ionization rates) of the two helium metastable states (2 1S and 2 3S) interacting with an aluminum surface are presented. They are used to study the singlet to triplet conversion process proceeding via He+ formation in thermal collisions of helium metastable atoms with metal surfaces. The conversion mechanism via He+ formation is found to be efficient for the larger workfunction surfaces, whereas, for lower workfunction surfaces, the conversion mechanism via He− formation which we studied earlier [Surf. Sci. 284 (1993) 337] is dominant.

Singlet to triplet conversion in low energy metastable helium-metal surface collisions: Auger deexcitation process

Abstract Experimental studies of metastable He atom collisions on alkali coated metals revealed that incoming He∗ (2 1 S) atoms are very efficiently converted into He∗ (2 3 S) atoms far from the surface. Various conversion processes have been proposed. This work concerns the Auger conversion process proposed by Woratschek et al. [Phys. Rev. Lett. 55 (1985) 611]. The non-perturbative coupled angular mode (CAM) method is shown to be able to treat Auger deexcitation processes. The predictions for the Auger conversion are shown to be quite consistent with those previously obtained for the He − intermediate process [Borisov et al., Surf. Sci. 284 (1993) 337]. This is interpreted by comparing the two processes which are shown to have the same physical basis. The conditions of applicability of the two approaches (resonant intermediate and Auger) are discussed. The importance of the singlet to triplet conversion for helium atoms colliding on low work function surfaces is confirmed.

Multi-electron and multi-state effects in the charge transfer processes between oxygen and metal surfaces

Abstract Oxygen atoms and molecules and their negative ions have an electronic structure containing a few equivalent outer electrons. In the course of an atom (molecule)-metal surface collision, each of them can participate in charge transfer processes. It is shown that this multi-electron aspect has to be taken into account, it generates multi-state and polarization effects. The consequences for the resonant charge transfer process are discussed and two examples are discussed: polarization effects in sputtering of O− ions and quenching of oxygen singlet states. A generalisation to other systems and electronic structures is presented.

Li+ neutralization as a probe of the local electronic potential in Li+-alkali covered metal surface collisions

Abstract A theoretical study of Li + neutralization by collision on an alkali covered Al surface is presented. The neutralization probability is computed for Li atoms back scattered from Al sites and from alkali sites on the surface. The calculations are performed with a model representation using lithium as a representative for alkali adsorption. The results reproduce the very large difference in neutralization probability for Li scattered from substrate and adsorbate sites experimentally observed by Weare and Yarmoff [Surf. Sci. 348 (1996) 359] and thus confirm the importance of local effects in atom-surface charge transfer. For scattering from adsorbate sites, the neutralization is shown to be associated with a very large excitation probability of the scattered Li atom.

Singlet levels of an He atom interacting with an Al surface: Interaction between quasi stationary states in the n = 2–6 manifolds

Abstract The excited singlet states (n = 2–6) of He atoms interacting with an aluminum surface are studied with the Coupled Angular Mode (CAM) method, which provides their energy positions and widths as functions of the atom-surface distance. The results show very strong couplings between the various states inside each manifold, induced both by direct couplings and indirect couplings via the metal states. These couplings can lead to an attraction as well as to a repulsion of the interacting states. The state mixing results in the formation of hybrids with a rather small width.

Auger neutralization and de-excitation of helium at an aluminium surface: a unified treatment

The neutralization rate of He+ and the de-excitation rates of excited He atoms in front of an Al surface are computed. This is achieved by evaluating the transition of electronic eigenstates of the He+‐Al system to the He+ core hole while producing surface excitations. These surface excitations are computed in a self-consistent way. The present work accounts for the full many-body excitation of the metal surface as well as the hybridization of the He‐Al orbitals. This hybridization leads to a new qualitative picture of the Auger processes in which the neutralization and de-excitation processes are mixed.