1 - Introduction

Summary

What is XAFS?

X-ray Absorption Fine Structure (XAFS) spectroscopy is a unique tool for studying, at the atomic and molecular scale, the local structure around selected elements that are contained within a material. XAFS can be applied not only to crystals, but also to materials that possess little or no long-range translational order: amorphous systems, glasses, quasicrystals, disordered films, membranes, solutions, liquids, metalloproteins – even molecular gases. This versatility allows it to be used in a wide variety of disciplines: physics, chemistry, biology, biophysics, medicine, engineering, environmental science, materials science, and geology.

The basic physical quantity that is measured in XAFS is the X-ray absorption coefficient µ(E), which describes how strongly X-rays are absorbed as a function of X-ray energy E. Generally µ(E) smoothly decreases as the energy increases (approximately as 1/E³), i.e. the X-rays become more penetrating. However, at specific energies that are characteristic of the atoms in the material, there are sudden increases called X-ray absorption edges. These occur when the X-ray photon has sufficient energy to liberate electrons from the low-energy bound states in the atoms. The cross section, a quantity that is proportional to µ(E), is shown in Figure 1.1 for the element platinum. Experimental data for MnO and KMnO₄ are shown in Figures 1.2 and 1.3.

Absorption edges were first measured in 1913 by Maurice De Broglie, the older brother of quantum mechanics pioneer Louis De Broglie.