PHYSICAL REVIEW A 83, 032901 (2011) Energy loss of keV fluorine ions scattered off a missing-row reconstructed Au(110) surface under grazing incidence L. Chen, 1,2 J. Shen, 1 J. E. Vald´ es, 3 P. Vargas, 3 and V. A. Esaulov 1,* 1 Institut des Sciences Mol´ eculaires (Unit´ e Mixte de Recherche CNRS Universit´ e, UMR 8214), b ˆ at 351, Universit´ e de Paris Sud, Orsay F-91405, France 2 School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China 3 Department of Physics, Universidad T´ ecnica Federico Santa Mar´ ıa, Valpara´ ıso, Casilla 110-V, Chile (Received 1 October 2010; published 2 March 2011) A joint experimental and theoretical study of energy loss is presented for 1-to-4-keV fluorine negative ions in grazing scattering on a missing-row reconstructed Au(110) surface. Measurements of energy losses for various azimuthal orientations of the crystal have been performed by means of a time-of-flight method with a pulsed beam. The dependence of the fraction of surviving negative ions on azimuthal angles, was determined. Our energy-loss data are discussed in light of trajectory and stopping-power calculations, where the explicit inclusion of the nonuniform electron density at the surface provides good agreement with the experimental data. The simulation allows us to delineate various trajectory classes that correspond to different contributions in the energy-loss spectra for various azimuthal orientations of the surface. DOI: 10.1103/PhysRevA.83.032901 PACS number(s): 34.50.Bw, 79.20.Rf I. INTRODUCTION Energy-loss phenomena of energetic ions in matter have attracted much attention because of their importance in fundamental research as well as in technological applications as an analytical tool. In the low-velocity regime (v  v 0 = bohr velocity), particles lose energy when they interact with matter due to elastic loss (that is, collisions with screened nuclei without any excitation) and inelastic loss (that is, electronic energy loss via electron-hole pair generation). Both energy- loss mechanisms compete depending on impact parameter and particle mass. For collisions in surface grazing scattering and in channeling conditions [1–3], projectiles collide in a sequence of correlated small-angle or large-impact-parameter events. As opposed to small-impact-parameter collisions, where inner shell excitations can occur leading to large energy losses in a single binary encounter [4], here energy losses in individual encounters remain small. Also in this case the elastic loss does not make an important contribution to the total particle energy loss. This condition is easily met by the steering of the projectile between neighboring atomic strings (axial surface channeling) or planes (semiplanar surface channeling). Both channeling phenomena occur when the projectile is scattered from a surface in grazing incidence, with angles less than the Lindhard critical angle [5]. In the surface channeling regime, the energy transfer of the projectile to lattice atoms of the crystal is very small because of large impact parameters so that electronic energy-loss phenomena can be directly investigated [5–7]. The description of ion slowing down during channeling in crystals and grazing scattering on surfaces is complicated because of inhomogeneity in the electronic density. In trans- mission experiments, electronic ion energy loss is greatly reduced in channeling orientations compared to that seen with a random orientation [8]. The case of ion-surface scattering is particularly complex because regions far from the first * Corresponding author: vladimir.esaulov@u-psud.fr atomic layer, where the electronic density decreases, contribute to energy losses. The trajectory lengths depend on the scattering conditions, such as incident and azimuthal angles, and one must correctly account for the spatial inhomogeneity, or corrugation, of the electron density in the crystal and above the surface. Several recent experimental and theoretical investigations have focused on these problems [5,6,9–18], but in these, an averaged electron density for a given distance from the surface is assumed, which is a poor description of the electron density corrugation above a crystal, especially in high-symmetry directions. Thus, for instance, we found it impossible [18] to fit our data for energy losses on Ag(110) over an extended energy range because very different trajectory types are found. However, we have recently developed an approach to describe slowing of ions through solids under conditions of strongly varying electron densities. We have applied it to the case of proton scattering through crystals [19] and in scattering on the Ag(110) surface [20]. In this paper, we report the main features of an experimental and theoretical study of the energy losses of low-energy fluorine ions scattered under grazing incidence on a Au(110) surface for various crystalline directions. We chose fluorine, because energy losses of slow heavy ions have not been the object of very detailed investigations as opposed to light-ion scattering. The Au(110) surface was chosen because it displays a missing-row reconstruction and is thus highly corrugated. Taking correctly into account the inhomogeneity of the surface electronic density seemed essential. This case thus provides a good testing ground for our model. Our experiments indeed showed very strong variations in the characteristics of energy losses when scattering along different azimuthal directions along the surface, where the ions encounter very different electron density contours. Our approach allows us to account for these variations and we are thus able to analyze these energy losses in terms of the ion trajectories and the electron densities encountered. A very brief preliminary account of some results for 4 keV was presented elsewhere [21], along with a description of the theoretical approach we use. 032901-1 1050-2947/2011/83(3)/032901(10) ©2011 American Physical Society