X-Ray spectroscopy involves a process in which an X-ray beam is applied to an atom and causes the ejection of an electron, usually a core electron. This leaves a vacancy in the core shell, and results in relaxation of an outer electron to fill that vacancy. This phenomenon is only observed when the energy of the X-ray exceeds the ionization energy of the electrons in that shell. We can relate this occurrence to the X-ray Absorption coefficient, which becomes the basis of EXAFS theory. The X-ray Absorption coefficient, or μ, describes the relationship between the intensity of an X-ray beam going into a sample and its intensity leaving the sample after traveling a distance x within the sample. The absorption coefficient is given by
μ=dlnI/dx