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(>5 eV apart), only one is used in quantitative analysis; otherwise, the collective area of both peaks should be used. The S(2pi/2,3/2) lines (unresolved) and the Sn(3d3/2,5/2) lines (resolved) in Figure 4b on p. 476 A illustrate examples of this phenomenon. Photoelectrons may also gain or lose energy at the site of photoemission through shake-off or shake-up processes (common for some transition metal compounds). From Equations 2,3, and 6, it is clear that for homogeneous materials where no standards exist, reasonable quantitation might still result if good estimates can be made for the variables in those relationships. However, the analyst must be assured of four things. First, the probability of ionization of the Auger or photoelectron or a corresponding relative yield factor must be available or calculable, and reasonably accurate. Second, good descriptions are needed to quantitate electron backscattering processes in solids. Third, the electron transport (energy loss) processes in solids that determine the escape depth must be well described. Finally, the analyzer transmission efficiency and the detector efficiency must be quantitatively described over the entire kinetic energy range of interest. Determinate (unidirectional) errors in any of these parameters can be expected. These errors will prevent the analyst from determining absolute concentrations of the analyte, but relative atomic ratio or atomic percentage calculations will generally cancel most of these errors. Factors that affect peak shapes in AES and XPS Now the question becomes, "How does the analyst obtain accurate peak areas in both AES and XPS, with which to proceed toward quantitation?" This critical question, if answered properly, can lead to good quantitative characterization of many solid surfaces (2128). Unlike many atomic emission spectroscopies where the line shapes are relatively simple, the detected peaks in either photoelectron or Auger spectroscopy of solids can often be obscured by spectral background contributions from a large number of other electron emission events (described below) in the near-surface region of the sample, making peak area determination difficult. Some of these are shown schematically in Figure 2. Both Auger and photoelectrons may lose energy at the emission site—an intrinsic energy loss process seen mainly in free electron metals—because of the relaxation of conducting electrons accompanying photoemission. It is also possible that
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472 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
they lose energy away from the emission site, before escaping the solid—an extrinsic energy loss process. This energy loss is attributable to energy exchange between the emitted electron and the population of conducting electrons in the near-surface region of the solid {21). This last process includes the so-called plasmon loss processes that actually lead to new peaks, displaced up to 40 eV below the kinetic energy of the parent Auger or photoelectron peak (most pronounced for metal systems). All of these energy losses are the factors that limit the sampling depth of XPS or AES in solids (13). The spectral background in AES arises from the following: • electrons originally at the kinetic energy of the source (typically 2000-5000 eV) that have been scattered from surface and subsurface atoms and lost energy in the process, • electrons arising from Auger emission events of higher kinetic energy than for the element of interest, • asymmetry in the spectral line shape (to the low kinetic energy side of the peak) because of the aforementioned "intrinsic" energy loss processes at the point of origin of the Auger electron in the solid, and • "extrinsic" energy loss processes that arise from scattering events occurring between the subsurface point of origin of the electron and the solidvacuum interface (29,30). For the first two processes, the population of scattered electrons builds exponentially to lower kinetic energies, resulting in a "secondary cascade" on which the Auger electrons are superimposed (30). The analytically relevant data may easily be less than 1% of the total secondary emission from the sample. This fact is one of the principal reasons why the detection limit for most elements using this technique is not better than 1 part per thousand. Because Auger events produce an electron whose kinetic energy is independent of the source energy, the spectral properties of the electron beam or Xray source that create the Auger electron do not enter into the final spectrum. The resolution and transmission efficiency of the analyzer and the energy-dependent response of the detector, however, do play a role in determining the spectral shapes and relative intensities for AES data. These complications are so extreme that for years AES was exclusively conducted using electrostatic analyzers whose pass energies were modulated, and Auger spectra were only presented in first-derivative form, after lock-in amplifier demodulation. The pitfalls of this approach to
