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Synchrotron radiation has a number of special characteristics that make electron storage rings and synchrotrons powerful tools for research with photons. Applications in both basic research and technology are possible and there are exciting prospects for the future with the design and construction of electron storage rings dedicated to synchrotron radiation production. Synchrotron radiation has a continuous spectrum ranging from the infrared to X-ray wavelengths. It is both intense and strongly polarized in the plane of the electron orbit. The light is emitted in pulses that are typically 1 × 10-9 sec long and have repetition frequencies of the order of 1 × 106 pulses per second. With the current designs, beam cross sections of 1 mm2 or less can be achieved and the photons are sharply collimated in a narrow cone about the tangent to the orbit of the emitting particle.

Since the first observations of the variation of the absorption coefficient for X-rays above the energy thresholds in the thirties until the early seventies, measurements and analysis of these variations were merely intended for the understanding of the underlying physics. Recently, with the understanding of the information available about the local atomic structure in the neighborhood of the absorbing species and the availability of high intensity synchrotron radiation sources, EXAFS has become a powerful structural tool. In the discussions that follow the details of the measurement for very dilute species will be presented. It is shown that for the more dilute systems the measurement of the emission rather than the direct absorption is a more favorable technique.