Introduction: Historical Perspective on XAS

This introductory chapter provides a brief sketch of the main events that have established x-ray absorption spectroscopy (XAS) and related techniques as leading scientific characterization tools in the period 1913-1971 (53 years). The first theoretical attempt to explain the fine structure in the XAS spectra was proposed in 1931 and 1932 by Kronig, who developed a model based on the presence of long-range order in the probed system. The new approach explained the x-ray absorption fine structure (XAFS) features in terms of modulation of the wave function of the final state of a photoelectron upon its scattering from the potentials of neighboring atoms. From an experimental point of view, a fundamental improvement in the instrumentation was achieved in the 1960s when commercial diffractometers were modified so that absorption spectra of much better quality could be obtained, though still using conventional x-ray tubes as a source.