As described in Chapter 2, a contaminant layer may form on a solid surface either by the surface interacting with the environment or by the bulk contaminant diffusing through the solid itself. Thin contaminant layers, such as adsorbed gases, water vapor, and hydrocarbons of atomic dimensions (approximately 2 nm thick), are unavoidably present on every surface of any solid that has been exposed to air. Surface analysis techniques, particularly x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), are well suited for examining these thin contaminant layers. However, contaminant surface layers can affect the spectrum by attenuating the electron signal from the underlying surface, thereby masking spectral features related to the bulk material [3.1–3.3].