Effect of oxygen adsorption on the energy losses in grazing scattering of hydrogen ions on Ag(1 1 0) J.E. Valde´s a, * , P. Vargas a , L. Guillemot a,b , V.A. Esaulov b a Departamento de Fı´sica, Universidad Te´cnica Federico Santa Marı´a, Valparaı ´so, Casilla 110-V, Chile b Laboratoire des Collisions Atomiques et Mole´culaires, (Unite´ Mixte de Recherche UMR 8625), baˆt 351, Universite´ de Paris Sud, Orsay 91405, France Available online 26 January 2007 Abstract In this work we present a study of the effect of oxygen adsorption on the energy loss of 4 keV hydrogen ions, which are scattered off an Ag (1 1 0) single-crystal surface with varying coverages of oxygen. In this case oxygen adsorption leads to an added row reconstruction of the surface. We performed measurements for grazing angles as a function of crystal azimuthal orientation, which show large differences in energy losses. Experimental results are discussed in the light of trajectory calculations of protons scattered under grazing incidence conditions on the surface. Using non-linear models for stopping power, ab initio crystal structure calculations of the electronic density and semi-classical simulations, we obtain data that is in very good agreement with experimental results. These simulations in particular allow us to properly take into account the variations of the surface electronic density and hence obtain an accurate description of the energy loss processes for ion scattering along various azimuthal orientations of the target. Ó 2006 Elsevier B.V. All rights reserved. PACS: 34.50.Dy; 34.50.Bw; 61.85.+p Keywords: Stopping power; Surface channeling; Ion scattering 1. Introduction Over past years considerable effort has been made to understand the characteristics of the stopping of ions in matter [1–3]. In recent years a number of investigations have focussed on the question of energy losses undergone by ions scattered on surfaces [4–11]. Here the problem is rendered particularly complex, because regions above the first atomic layer of the surface, where the electronic den- sity decreases, contribute to energy losses. Several attempts have been made to extract ion-surface distance dependent stopping powers and draw conclusions about stopping in an inhomogeneous electron gas [4–11]. A correct theoreti- cal description of experimental data however requires a realistic description of the density corrugation above a crystal and particle trajectories. In the present paper, we investigate the effect of adsorb- ing oxygen on energy losses of hydrogen ions scattered under grazing incidence on an Ag (1 1 0) surface for various crystalline directions. Oxygen adsorption on Ag(1 1 0) leads to an n · 1 added row reconstruction which at the highest – saturation-coverages corresponds to a 2 · 1 structure (see e.g. [12,13] and Fig. 1(a)). In this case it has been shown that oxygen atoms are located between two adjacent silver atoms in an added row. This case thus presents the possibility of studying energy losses under well defined and different cov- erage conditions and different surface reconstructions. In the following, we shall briefly outline the experimen- tal approach and then present a modelling of the experi- mental results using computer simulations following an approach developed by us previously [14]. This approach allows us to study the differences induced by lattice steering effects in the energy loss distributions and most impor- tantly properly take into account the inhomogeneity of the surface electronic density. We can thus obtain an accurate 0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.11.114 * Corresponding author. E-mail address: jorge.valdes@usm.cl (J.E. Valde´s). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 256 (2007) 81–85 NIM B Beam Interactions with Materials & Atoms