Elements of XAFS

Introduction Over the last two decades X-ray Absorption Fine Structure (XAFS) spectroscopy has emerged as an incisive probe of the local structure around selected atomic species in solids, liquids, and molecular gases. Foremost among its strengths are its applicability to amorphous materials and its \tunability" { the ability to probe the environments of di erent elements in the sample by selecting the incident x-ray energy. Although the phenomenon and its basic explanation in terms of a quantum mechanical interference e ects have been known since the 1930's, the phenomenon did not become a practical experimental tool until two events occurred: the distillation by Stern, Sayers, and Lytle of the essential physics of the process into the standard XAFS equation and their proposal of a simple method of data analysis; and the availability of tunable, high ux, high energy-resolution synchrotron radiation beamlines. The number of XAFS experiments performed has grown exponentially since 1970 as a consequence of these developments. The amount of information available from a single XAFS spectrum (typically 10-20 or so parameters) is small compared to that available from x-ray di raction, but the information available from a well-chosen experiment can be particularly incisive and may be inaccessible by any other technique. XAFS (a short-range order technique) is particularly powerful when intimately combined with complementary techniques such as x-ray di raction (a long range order technique). Recent developments in theory and experiment show great promise for extending the range and power of the XAFS. The purpose of this article is to provide a basic introduction to XAFS as it has been traditionally practiced and to provide a brief survey of important recent advances in the eld.