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An overview is given of improvements in the reliability of X-ray photoelectron spectroscopy (XPS) measurements over the past three decades. Early inter-laboratory comparisons showed wide variations in measurements of peak energies and relative peak intensities for the same material. Procedures now exist for calibration of instrumental energy and intensity scales. It was generally necessary in early work to assume that the surface was homogeneous over the XPS sampling depth. It is now possible to assess the extent of compositional inhomogeneities in depth with angle-resolved XPS measurements or from analyses of the photoelectron energy distributions in the vicinity of major peaks. The surface sensitivity of XPS was assumed in early work to be given by the so-called "universal curve" for effective attenuation lengths (EALs) as a function of electron energy. Calculations of electron inelastic mean free paths (IMFPs), however, show appreciable material dependencies. In addition, elastic scattering of the signal electrons is often significant. As a result, the EAL can be appreciably different from the corresponding IMFP. Brief mention is also made of the development of NIST data resources for XPS: an XPS database, IMFP and EAL databases, and standard test data for XPS. The standard test data for XPS are a useful teaching tool for estimating uncertainties of peak parameters for overlapping peaks in an XPS or other spectrum.
C. J. Powell (Wed,) studied this question.