Numerical treatment of electron-energy-loss spectra and determination of optical constants

Abstract

Elcctron-cncrgy-loss spectr;i in the reilexion mode (RFF.LS) have been obtained for palladium and silicon oxide and the data treated numerically in order to derive their optical constants f-:\ and f:^. The numerical method consists of obtaining the loss function Im(-l/ir) from llie experimental RHHLS spectra, and then, through a second slep of numerical inlegration, accessing the real part of ( -l/'t) by Kramers Kronig analysis. if I. INTRODUCTIONElcctron-cncrgy-loss spectroscopy (EELS) is often used to characterize material surfaces. It was widely and successfully used in our previous works (Ghamnia el al. 1996, 1997, Bouslama et a!. 2002 for studying the electronic properties of solids. We usually use it as a complementary method to Auger electron spectroscopy (AES) for material surface analysis in which the same apparatus is used for AES and EELS. REELS spectra are rich in other information related to the optical properties of the material investigated. Some workers have already used energy-loss spectroscopy in the transmission or refleetion mode (Rowe and Ibach 1973. Raether 1980, Laine el al. I98S and Yebero el al, 1990 to determine optical constants. Here we treat the REELS spectra numerically to determine the energyloss function, and then the real and imaginary parts of the dielectric constant using Kramcrs-Kronig analysis. We have divided the procedure of treating the data into two steps, as reported below. § 2. NUMHRICAI.TRKATMHNT OF SPHCTRA 2.1. Slep I The first step involves the separation of the elastic peak and the subtraction of the background signal from the experimental REELS spectrum. The elastic peak is taken as the energy origin. For metals, errors may be introduced because the elimination of the elastic peak leads to the loss of information in the low-energy range between 0 and 4eV. In fact, the first structures generally appear near the beginning of the elastic peak. This is not the case for insulating materials since the band gap is tAuthor for correspondence.