A 6 1 4

Surface Science 292 (1993) 317-324 North-Holland

N20 adsorption and decomposition at a CAO(100) surface, studied by means of theory Anders Snis, Dan Str6mberg and Itai Panas Department of Inorganic Chemistry, University of G6teborg, S-412 96 G6teborg, Sweden

Received 13 April 1993; accepted for publication 4 May 1993

The adsorption and decomposition of an NzO molecule at different sites on a CaO(s) surface are investigated by means of ab initio quantum chemistry. The calcium, Ca 2+, and oxygen, O ~ - , sites at a perfect (100) surface and at a corner position, 0 2 - , are considered. Adsorption energies at different sites are calculated and the largest value, 6 kcal/mol, is obtained for a corner site. The barrier for dissociation is calculated to 26 and 27 kcal /mol at the 0 2 and 0 2 - sites, respectively. These values are some 10 kcal /mol lower than the experimental estimate, and the discrepancy is understood from methodological difficulties to describe the free N20 molecule. A mechanism for the dissociation over an 0 2- site is proposed, whereby the transfer of the O atom goes via a linear N - N • •. O • • - O 2- transition state.

Surface Science 292 (1993) 325-341 North-Holland

Study of H- formation in grazing surface collisions Rainer Zimny Institute of Nuclear Physics, University of Miinster, W'dhelm-Klemm-Strasse 9, D-4400 Miinster, Germany

Received 16 October 1992; accepted for publication 13 April 1993

By combining the transfer Hamiltonian (TH) formalism with a rate-equation approach we have studied the final formation of negative hydrogen ions in grazing scattering of fast protons at the surface of a free-electron metal. In particular, the influence of the image potential on the dynamics of H - formation has been investigated. Good overall agreement of our model calculations with recent experimental data for an AI ( l l l ) surface is found.

Furthermore, we have derived a simple analytical relation of the H - fraction in the scattered beam with the particle velocity parallel to the surface, the surface work function and the effective binding energy of the H - ion, respectively. This formula may be used as a tool to analyze experimental data. In particular, it allows a reliable estimate of the effective atom-surface distance of final H - formation along the outgoing path of the projectile in grazing ion-surface scattering experiments. The sensitivity of resonant transfer rates and H - formation probabilities, respectively, on the shape of the surface-potential harrier is also studied.

Surface Science 292 (1993) 342-348 North-Holland

Low-energy electron diffraction calculations using a parallel supercomputer Wayne K. Ford lntel Corporation AL3-15, 5200 NE Elam Young Pkwy, Hillsboro, OR 97124, USA

Received 15 January 1993; accepted for publication 5 April 1993

A first implementation of a dynamical low-energy electron diffraction theory on a parallel supercomputer is described. The computational performance of the Duke-Laramore-Beeby LEED formulation is demonstrated using an Intel iPscTM/860 parallel computer. The algorithm developed is highly parallelizable and scales nearly ideally with the number of nodes used. A performance equivalent to that experienced using a Cray YMP was attained with as few as 16 nodes. However, it was not possible to exceed significantly the YMP performance due to the increasing inter-node communication overhead. A simple mathematical model is described to explain this limitation, which is encountered in all parallel computing applications and hardware designs. The model indicates that an alternate parallelization scheme is required to attain greater performance.