In the presented work we performed an experimental study of the work function decrease for a silicon sample caused by Cs+ ion bombardment. We varied the energy of primary Cs+ ions as well as the angle of incidence in order to reach a different concentration of implanted cesium ions and to find a dependence of the work function change on the cesium surface concentration. A "surface dipoles" model was developed. The model based on the electronegativity concept considers formation of Si-Cs dipoles with corresponding dipole moment. An electric field of these dipoles results in a decrease of the work function, whereas a partly ionic character of Si-Cs bond increases the surface binding energy of cesium and, as a consequence, its surface concentration. A good correlation between the model and the experimental data was found. (C) 2002 Elsevier Science B.V. All rights reserved.