SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2005; 37: 1151–1157 Published online 3 November 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.2126 Theoretical study of the surface excitation parameter from reflection-electron-energy-loss spectra N. Pauly, 1* S. Tougaard 2 and F. Yubero 3 1 Universit ´ e Libre de Bruxelles, Service de M ´ etrologie Nucl ´ eaire (CP 165/84), 50 av. F. D. Roosevelt, B-1050 Brussels, Belgium 2 Physics Institute, University of Southern Denmark, DK-5230 Odense M, Denmark 3 Instituto de Ciencia de Materiales de Sevilla, Isla de la Cartuja, E-41092 Sevilla, Spain Received 9 April 2005; Revised 30 August 2005; Accepted 30 August 2005 A theoretical method to determine the so-called surface excitation parameter (SEP) is presented. This method is based on the modelling of reflection-electron-energy-loss spectroscopy and more particularly on the analysis of energy-differential inelastic electron scattering cross sections calculated within the model. The SEP is extracted from theoretical cross-section spectrum by calculating the ratio between the surface loss component of the spectrum and the elastic peak intensity. The calculations have been performed entirely with the dielectric function, using the software QUEELS (Quantitative analysis of Electron Energy Losses at Surfaces) recently developed by Yubero and Tougaard [Surf. Interface Anal. 2004; 36: 824]. The angular distribution of SEP is calculated for angles between 10° and about 70° for aluminium and silicon. We propose also an extension of the method for materials (e.g. copper) that do not present clear surface and volume plasmons. Copyright  2005 John Wiley & Sons, Ltd. KEYWORDS: surface excitation; surface plasmon; REELS; Al; Si INTRODUCTION Precise information about inelastic scattering of electrons traveling within a surface region are particularly important for the widely used surface electron spectroscopies such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection-electron-energy-loss spec- troscopy (REELS). The inelastic mean free path (IMFP) of electrons is thus a key factor for surface analysis spectro- scopies. Optical data of solids 1 made it possible to obtain general predictive formulas for the IMFPs of solids, i.e. the TPP-2M of Tanuma, Powell and Penn 2 or the formulas of Gries. 3 On the other hand, another method 4 for the determination of the IMFPs is the Elastic Peak Electron Spectroscopy (EPES) 5 based on the measurement of the elastically backscattered electron peak intensity from surfaces. However, a compilation of both calculated and measured IMFP values published by Powell and Jablonski 6 indicates large deviations between theoretical and experimental data. It has been shown 7,8 that one of the reasons for this deviation originates from surface excitation effects, i.e. the surface plasmons, neglected in theoretical calculations. Consequently, it is important to improve the models for bulk and surface excitations to get a better knowledge of the IMFP L Correspondence to: N. Pauly, Universit´ e Libre de Bruxelles, Service de M´ etrologie Nucl´ eaire (CP 165/84), 50 av. F. D. Roosevelt, B-1050 Brussels, Belgium. E-mail: nipauly@ulb.ac.be Contract/grant sponsor: Universit´ e Libre de Bruxelles. Contract/grant sponsor: Fonds National de la Recherche Scientifique (FNRS). parameter. This is now the object of intense research (see 9–13 for instance). In general, the intensity of surface excitations is charac- terized not by a scattering cross section but by a probability, the surface excitation parameter (SEP) defined as the aver- age number of excitations an electron undergoes when it crosses the surface once. 14 A theoretical calculation of the SEP is rather complex. In order to evaluate more easily the effect of surface excitations, simple formulas for the SEP depending on the energy of the electron, the emission angle and one or two material-dependent parameters have been proposed. 7,11,13 The presence of material-dependent param- eters calculated 13 or obtained from fitting with experimental measurements 11 considerably limits the use of these simple formulations. In this work, we propose a new theoretical approach to calculate the SEP and its angular distribution on the basis of the model of Yubero, Sanz, Ramskov and Tougaard 15 to describe the energy losses in REELS. In this formalism, the SEP calculations are performed only with the help of the dielectric function of the considered medium and do not necessitate the introduction of another material parameter. Moreover, Tougaard and Yubero 16 have recently developed the software Quantitative analysis of Electron Energy Losses at Surfaces (QUEELS) allowing to conveniently perform calculations of not only electron energy losses in a REELS geometry but also for other cases of electrons moving near surfaces in general geometries. We show in Fig. 1 results for Si calculated for REELS inelastic scattering cross sections K sc ⊲E, ¯ hω⊳ compared with cross sections extracted from experimental data. This figure (already published in Ref. 17) Copyright  2005 John Wiley & Sons, Ltd.