X-ray absorption spectroscopy (XAS) involves the excitation of core electrons to bound states localized on the photoabsorber and the eventual excitation of the photoelectron to the continuum. The resulting spectra are typically divided into two regions: (1) the edge region which provides electronic structure information and (2) the extended X-ray absorption fine structure (EXAFS) region, which provides information about the distance, number, and type of ligands. Here, a basic introduction to XAS theory, the information that can be obtained, and the experimental consideration are presented. The application of XAS to biological systems and the impact this has had on nitrogenase research are briefly highlighted. New experimental advances are described.
