Channelling and stopping of hydrogen and fluorine ions in grazing scattering on an Ag(1 1 0) surface at low keV energies and effects of oxygen adsorption L. Guillemot a , V.A. Esaulov a, * , E.A. Sanchez b a Laboratoire des Collisions Atomiques et Mol eculaires (Unit e Mixte de Recherche UMR 8625), b^ at 351, Universit e de Paris Sud, Orsay 91405, France b Centro At omico Bariloche, Instituto Balseiro, CNEA, and CONICET, 8400 San Carlos de Bariloche, R  ıo Negro, Argentina Abstract We present measurements of energy-loss spectra of hydrogen and fluorine ions, which are scattered off an Ag(1 1 0) single-crystal surface for primary energies between 1 and 6 keV. The effect of oxygen adsorption, which leads to an added row reconstruction of the surface is investigated. We performed measurements for grazing angles in chanelling conditions. Experimental results are discussed in the light of trajectory calculations of hydrogen and fluorine atoms scattered under grazing incidence conditions on the surface. Different trajectory classes are found by the calculations and assigned to different contributions in the energy-loss spectra. These data on ion stopping allow us to deduce the values of friction coefficients as a function of atom–surface distance. Ó 2003 Published by Elsevier B.V. 1. Introduction The stopping of ions in matter has attracted much attention both theoretically and experimen- tally. Many experiments have been performed with charged particles penetrating bulk matter [1–3]. From a practical point of view this subject is of interest in a number of areas such as semiconductor doping using ion beams or hadron therapy – ion beam cancer treatment. In all these cases an accu- rate knowledge of penetration depth of ions in matter is necessary. In many cases stopping powers are deduced by dividing the measured energy losses by the trajectory length generally given by the target thickness. In the case of monocrystalline solids the situation is complicated by channelling. Two parts contribute to the energy loss of charged particles interacting with matter. The nuclear losses are of elastic nature and can be calculated from binary collision models. The electronic losses, which are mainly inelastic, are more difficult to describe. To do so, detailed knowledge of projectile and target properties is needed. In the case of sur- face scattering, the situation is even more compli- cated. The impact parameters are not limited by the target density, hence, effects taking place tens of a.u. in front of the surface may contribute to energy losses. The trajectory lengths are difficult to define and depend on the scattering conditions, like angle * Corresponding author. Tel.: +33-1-6915-7680; fax: +33-1- 6915-7671. E-mail address: esaulov@veof1.lcam.u-psud.fr (V.A. Esau- lov). 0168-583X/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0168-583X(03)01718-X Nuclear Instruments and Methods in Physics Research B 212 (2003) 20–26 www.elsevier.com/locate/nimb