XAS is a synchrotron based technique. This means that we shine UV or soft or hard x-rays on the sample of interest. The synchrotron allows us to tune this energy so that we can scan the photon energy accross one of the absorption edges of the material we are interested in. We measure and plot the absorption as a function of photon energy. The shape and postion of the absorption edges are characteristic of the element doing the absorbing and of the environment in which it is. For instance, the shape and position of the L3 edge of iron depends whether it is in its metallic form or one of its oxide forms. Here is an example of an XAS spectrum of iron:
XAS is an element specific probe that yields chemical information about the element of choice in a sample, including elements in burried layers of a multilayer structure.
When an photon is absorbed, an electron is ejected from the sample. There are different ways to measure the magnitude of the absorption. We can the current necessary to replenish the sample with electrons ( sample current method). We can capture the electrons that are ejected from the sample (partial electron yield method ). We can measure the incident versus transmitted photons if we have a thin enough sample ( transmission ). Or, after an electron is ejected it leaves behind a "hole." Electrons from higher energy levels of the atom "fall in these holes" and emit radiation in the process. We can measure this radiation and thus get a measurement of the initial absorption. This is the fluorescence method.