ENERGY CALIBRATION OF X-RAY PHOTOELECTRON SPECTROMETERS .2. ISSUES INPEAK LOCATION AND COMPARISON OF METHODS

An analysis is presented of issues involved in peak location for the c alibration of the binding energy (BE) scales of x-ray photoelectron sp ectrometers. These issues include the effects of peak asymmetry, the s urface core-level shift, and the avoidance of a sloping background whe n fitting spectra for energy calibration purposes. Examples of uncerta inty budgets for BE measurements are then presented in which illustrat ive values are shown for the repeatability standard deviation (for rep eated BE measurements of the same calibration peak), the expanded unce rtainty (at the 95% confidence level) for BE measurements following ca librations based on different numbers of peak measurements, and the to lerance for BE-scale drift and non-linearity for two chosen values (+/ -0.1 and +/-0.2 eV) of the total expanded uncertainty for a BE measure ment (at the 95% confidence level). It is recommended that a user prep are an uncertainty budget of this type to show clearly the sources of random and systematic error in BE measurements following a calibration . The reference data published by the UK National Physical Laboratory for BE-scale calibration were obtained from fits with a quadratic func tion to a group of points comprising the top 5% of each peak. Most com mercial x-ray photoelectron spectrometers have software available for spectrum synthesis, and we consider here the use of the commonly avail able Lorentzian, Gaussian, and asymmetric Gaussian functions for peak location. Illustrative fits with Cu 2p(3/2) spectra (measured with umm onochromated Al x-rays) showed that comparable accuracy and precision could be obtained with Lorentzian and Gaussian functions as with the q uadratic-equation method when different fractions of the peak were fit ted. For this asymmetrical line, the asymmetric Gaussian function allo wed better accuracy and precision to be obtained with a greater fracti on of the line than was possible with the symmetrical functions. (C) 1 997 by John Wiley & Sons, Ltd.