B. Bahrim

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 − Al surface. These were obtained with the coupled angular mode method and fully take into account the multielectron, multistate and polarization effects of the O − Al 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.

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.

Amorphous Ge quantum dots embedded in SiO2 formed by low energy ion implantation

Under ultrahigh vacuum conditions, extremely small Ge nanodots embedded in SiO2, i.e., Ge–SiO2 quantum dot composites, have been formed by ion implantation of Ge+74 isotope into (0001) Z-cut quartz at a low kinetic energy of 9keV using varying implantation temperatures. Transmission electron microscopy (TEM) images and micro-Raman scattering show that amorphous Ge nanodots are formed at all temperatures. The formation of amorphous Ge nanodots is different from reported crystalline Ge nanodot formation by high energy ion implantation followed by a necessary high temperature annealing process. At room temperature, a confined spatial distribution of the amorphous Ge nanodots can be obtained. Ge inward diffusion was found to be significantly enhanced by a synergetic effect of high implantation temperature and preferential sputtering of surface oxygen, which induced a much wider and deeper Ge nanodot distribution at elevated implantation temperature. The bimodal size distribution that is often observed in high...

Electron dynamics and level broadening in slow atomic interactions with metal surfaces and thin metallic films

Abstract Theoretical results on the electron dynamics and on resonant level broadening in slow atomic interactions with metal surfaces and thin metallic films are presented. Within the time-dependent close-coupling method, a wave-packet approach is introduced in which nondiagonal couplings in the subspace of metal states are taken into account. This approach is applied in a study of the time evolution of the occupation of hydrogen states in front of a semi-infinite metal. First-order level widths are calculated for hydrogenic atoms interacting with thin metallic films. They are found to exhibit pronounced size-quantization effects arising from the confinement of the electronic motion in the growth direction of the film.

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.

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.

Charge-transfer dynamics in slow atom–surface collisions: a new close-coupling approach including continuum discretization

Within a new two-center close-coupling expansion, we solve the time-dependent Schrodinger equation for an active electron interacting with a slow projectile and a metal surface. The continuum of conduction-band states corresponding to the motion of the active electron in the metal subspace along the surface normal is discretized in terms of wave packets. Results for the time evolution of the atomic and metallic population amplitudes for an excited hydrogen atom near an aluminum surface are shown and discussed.

Electronic excitation by electron impact of the O2 molecule physisorbed on a metal

The electronic excitation process by low energy electron impact is studied theoretically for the case of O2 molecules physisorbed on a model jellium metal (Al). The spin forbidden excitations to the a 1Δg and b 1Σ+g states are considered. Only the resonant contribution corresponding to the 2Πg O−2 resonance, which dominates at low energy in the free molecule is included in the present work. The characteristics of this resonant process involving a resonant state hidden below the excitation threshold are analyzed; in contrast with the free molecule case, a very important excitation process occurs below the energy threshold. The dependence of the excitation process on the symmetries of the problem is also discussed.

Local effects of the adsorbates on the resonant atom-surface charge transfer

Abstract A theoretical study of the charge transfer process between a collisional atom and an Al surface partially covered by alkali adsorbates is presented. The non-perturbative Coupled Angular Mode (CAM) method is applied to calculate the static properties of the projectile states. The energies and the widths of the projectile states are strongly modified in the vicinity of the adsorbate compared to the clean surface case. This is attributed to the molecularisation of the atomic and adsorbate levels. The dynamics of the neutralization of Li + ions is studied in the case of backscattering from the Al sites and from the alkali sites on the surface. Large differences in the neutralisation probabilities for Li + scattered from the substrate sites and from the adsorbate sites are obtained. The pecularities of Li − formation in backscattering from alkali adsorbate sites are discussed.

Electron transfer and orbital hybridization in slow collisions between excited hydrogen atoms and aluminum surfaces

Abstract We have developed a new two-center close-coupling approach in which the time-dependent Schrodinger equation is solved for an active electron interacting with a slow projectile and a metal surface. The motion of the active electron in the metal subspace is discretized by using Weyl wave packets. Results for the time evolution of the atomic and metallic population amplitudes for the H/Al system are shown and discussed.