Low-energy ion scattering at surfaces

Surface composition analysis by low-energy ion scattering

The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation The effective barrier potential includes quantum corrections to the classical image potential The effect of below-barrier (``tunneling'') transfer is investigated The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons The calculation describes the transient formation of ``hollow'' atoms We find satisfactory agreement with recent data for K Auger yields by Meyer et al [Phys Rev Lett 67, 723 (1991)]

Above-surface neutralization of highly charged ions: The classical over-the-barrier model

The UBI-QEP method: A practical theoretical approach to understanding chemistry on transition metal surfaces

The study of catalytic behavior begins with one seemingly simple process, namely the hydrogenation of O to H2O on platinum. Despite the apparent simplicity its mechanism has been much debated. We have used density functional theory with gradient corrections to examine microscopic reaction pathways for several elementary steps implicated in this fundamental catalytic process. We find that H2O formation from chemisorbed O and H atoms is a highly activated process. The largest barrier along this route, with a value of approximately 1 eV, is the addition of the first H to O to produce OH. Once formed, however, OH groups are easily hydrogenated to H2O with a barrier of approximately 0.2 eV. Disproportionation reactions with 1:1 and 2:1 stoichiometries of H2O and O have been examined as alternative routes for OH formation. Both stoichiometries of reaction produce OH groups with barriers that are much lower than that associated with the O + H reaction. H2O, therefore, acts as an autocatalyst in the overall H2O formation process. Disproportionation with a 2:1 stoichiometry is thermodynamically and kinetically favored over disproportionation with a 1:1 stoichiometry. This highlights an additional (promotional) role of the second H2O molecule in this process. In support of our previous suggestion that the key intermediate in the low-temperature H2O formation reaction is a mixed OH and H2O overlayer we find that there is a very large barrier for the dissociation of the second H2O molecule in the 2:1 disproportionation process. We suggest that the proposed intermediate is then hydrogenated to H2O through a very facile proton-transfer mechanism.

Catalytic water formation on platinum: a first-principles study.

Small angle multiple reflection of low energy (6keV) noble gas ions from single crystal surfaces as a means to study surface texture and contamination

Low-energy ion reflection from metal-surfaces

Charge-exchange of low-energy he ions and atoms scattered from a copper single-crystal

Electron emission induced by multiply charged Ar ions impinging on a tungsten surface

Study of low energy noble gas ion reflection from monocrystalline surfaces; influence of thermal vibrations of the surface atoms: I. Computer simulation of energy and spatial distributions

A.L. Boers

Papers

Small angle multiple reflection of low energy (6keV) noble gas ions from single crystal surfaces as a means to study surface texture and contamination

Low-energy ion reflection from metal-surfaces

Charge-exchange of low-energy he ions and atoms scattered from a copper single-crystal

Electron emission induced by multiply charged Ar ions impinging on a tungsten surface

Study of low energy noble gas ion reflection from monocrystalline surfaces; influence of thermal vibrations of the surface atoms: I. Computer simulation of energy and spatial distributions