D. Teillet-Billy

Energy shift and broadening of H levels in front of a metal surface

Abstract The energy position and width of H levels in front of an Al surface are studied with the nonperturbative coupled angular mode (CAM) method. The results are compared with previously published results by Nordlander and Tully [Phys. Rev. Lett. 61 (1988) 990 and Surf. Sci. 211/212 (1988) 207]. A strong hybridization is observed inside the various manifolds. The angular distributions of ejected electrons associated with the hybrids of the n = 2 and 3 manifolds are analyzed and correlated with the size of the level widths.

Singlet-to-triplet conversion in low energy metastable helium-metal surface collisions

Abstract Metastable de-excitation spectroscopy studies of alkali coated metals revealed the existence of a very strong conversion of the He(21S) metastable into the 23S metastable state occurring prior to the metastable de-excitation on a low work function surface. Hemmen and Conrad [Phys. Rev. Lett. 67 (1991) 1314] and Brenten et al. [Z. Phys. D 22 (1992) 563] suggested that the formation of a He− (1s2s22S) resonance state could be an efficient intermediate step in this conversion. Using the coupled angular mode method, we study the energy position and widths of a He − ion in front of a metal surface. This allows us to study quantitatively this conversion mechanism. It is found to be very efficient and to be associated with a strong electron emission corresponding to the decay of the He− ion to yield a ground state He atom.

Resonant electron scattering by molecules adsorbed on a surface: N2-Ag system

Abstract A model study of resonant electron scattering by static molecules adsorbed on a metal surface is presented, using the recently developed coupled angular mode (CAM) method. It is applied to the case of N 2 molecules adsorbed on an Ag surface. The N 2 −2 π g resonance characteristics (energy position and width) are determined and shown to be modified by the presence of the surface in qualitative agreement with the experimental results of Demuth et al. [Phys. Rev. Lett. 47 (1981) 1166].

H− formation in the scattering of hydrogen ions on an Al surface

Abstract We present results of an experimental and theoretical investigation of H − formation in collisions of 1–4 keV positive and negative hydrogen ions with clean Al surfaces. The scattered ion fractions were investigated in a large angular range extending from 2 to 40° with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process in a wide range of collision velocities normal to the surface, allowing us in particular to reach the domain where small atom-surface distances play an important role. Results of theoretical calculations could thus be tested in a more stringent manner than in previous high grazing-incidence experiments of Wyputta et al.

Image states on a free-electron metal surface covered by an atomically thin insulator layer

Abstract Excited electronic states on a free-electron metal surface are investigated in the case when an atomically thin Ar layer is adsorbed on the surface. It is shown that the dielectric character of the Ar layer allows the existence of image states as well-defined resonances in this system. The image states are confined in vacuum by the Ar layer and decay by electron transmission through the Ar layer. A 3D microscopic description of the Ar layer associated to a wave-packet propagation approach is used to determine the characteristics, energy and lifetime, of the image states in a model Ar/free-electron metal system. The possibility of the observation of these states in a time-resolved experiment is discussed.

Angular distributions in resonance electron scattering by oriented physisorbed molecules: O2Ag(110)

Abstract We present an experimental and theoretical study of the angular distribution of electrons resonantly scattered by physisorbed O 2 Ag (110) . The shape of the angular distribution, obtained from high-resolution electron energy-loss spectra (HREELS), changes as the coverage is increased from the monolayer to the multilayer regime. With the aid of a classical model which takes into account both the azimuthal and polar orientation of the molecule with respect to the surface and employs structural parameters deduced from X-ray absorption experiments, we find that this result can be attributed to a change in orientation of the molecules in the multilayer compared with the monolayer regime.

Resonant electron capture in atom metal collisions: HAl(111)

Abstract Resonant electron capture in fast grazing HAl(111) collisions is studied with a new nonperturbative method, the coupled angular mode (CAM) method. The quantities needed to describe the electron capture, namely the energy position and autodetachment width of the H − negative ion state are determined. This static study, in contrast with an earlier perturbation treatment, is in agreement with data extracted from experimental results on electron capture by Wyputta et al.

Non-adiabatic couplings in resonant charge transfer during atom-surface collisions

Abstract The formation of the metastable (1D and 1S) negative ion levels associated with the 2p4 electronic configuration of nitrogen is theoretically investigated in the course of collisions on an aluminium surface with alkali adsorbates. The multi-electron and multi-state aspects due to the presence of a few quasi-equivalent electrons in nitrogen are shown to lead to a complex structure of the resonant charge transfer (RCT) process. The RCT between the metal target and the nitrogen is deeply influenced by the presence of alkali adsorbates on the surface. The interaction between the N− ion levels and the adsorbate level leads to the possibility of non-adiabatic transitions induced by the collision movement. A diabatic modelling is presented, allowing an easy treatment of these non-adiabatic transitions and an evaluation of their importance. These theoretical results are then used for a discussion of the experimental results by Muller et al. [H. Muller, R. Hausmann, H. Brenten, V. Kempter, Surf. Sci. 303 (1994) 56]. They can account for the unexpected experimental observation of a sizeable electron emission due to the decay of free N−(1D) ions formed by collisions on alkali-covered metal surfaces.

The N−2(2Πg) shape resonance in slow collisions with metallic surfaces

Abstract Metastable N 2 ( A 3 Σ + u ) molecules colliding with a metal surface can be de-excited via an Auger process in which the excitation energy of the molecule is given to an electron of the metal. We report on a theoretical study of this process for N 2 ( A 3 Σ + u ) molecule collisions on a free electron metal, Al. This de-excitation process is efficiently mediated by the formation of a transient negative molecular ion, in the present case, the well-known resonance N − 2 of 2 Π g symmetry. A modelling of the electron–N 2 molecule interaction in the intermediate 2 Π g symmetry allows the computation of the Auger de-excitation process characteristics: transition rate, emitted electron spectra. The present results are used to discuss the experimental results on the N 2 ( A 3 Σ + u ) de-excitation in thermal collisions on metal surfaces by Stracke et al. [Surf. Sci. 396 (1998) 212] who first proposed this de-excitation mechanism. We find this resonance mediated process to be very efficient.

Theoretical and experimental study of N- (1D) formation in nitrogen collisions with a metal surface

The dynamics of nitrogen collisions with metals partially covered by alkali atoms is studied both experimentally and theoretically. Our attention focuses on the formation of N−(1D) metastable ions and their interaction with the surface. We present the electron energy spectra induced by slow collisions of N+ ions with partially cesiated Pd(111) surfaces under grazing incidence. These spectra display, as a function of Cs coverage, a sharp feature which is due to the autodetachment of N−(2p4, 1D) to the N(2p3, 4S) ground state. Our calculations, performed with the coupled angular mode (CAM) method on the basis of the resonant electron exchange between the nitrogen atom in states of the 2p3 configuration and the metal surface, consistently explain how negative ions formed close to the surface can survive against electron loss to the metal during the outgoing trajectory and can later decay as free ions. In order to understand the alkali coverage dependence of the N−(1D)-N(4S) peak intensity, the local character of the nitrogen interaction with the surface partially covered by adsorbate atoms has been taken into account.