3. Synchrotron radiation-based X-ray absorption techniques

#### 3.1. Hard X-ray absorption spectroscopy (HXAS)

X-ray absorption spectroscopy (XAS) is a powerful technique that can characterize aII forms of matter, irrespective of their degree of crystallinity. Traditionally, diffraction-based characterization methods are being used for structural investigations, and reliable structural information can be determined for materials that exhibit a long-range structural order. In contrast XAS can probe the local structure of disordered solids, liquids, as well as amorphous materials. XAS has vast application area ranging from coordination chemistry, catalysis, biology, and surface physics to material chemistry. One of the major advantages of XAS is its atomic selectivity which makes it possible to study the local structure of each different constituent of a sample. Sample preparation for XAS is very simple, and experiments can be performed in situ.

XAS spectrum can be divided in two parts, namely, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). In XANES phenomenon, an element-specific signal is generated, typically using a synchrotron radiation source. A core electron absorbs the energy of incident X-rays and gets excited beyond the Fermi level, leaving behind a core hole. The synchrotron radiation sources can provide energy that is right for desired electron transitions. When a sample is exposed to X-rays, it will absorb part of the incoming photon beams. Other phenomena occurring are heat, X-ray fluorescence, production of unbound electrons, and of course the scattering of X-rays. The absorption of X-rays can be measured quantitatively, and it follows exponential decay given by the Beer's law [10].

The EXAFS phenomenon arises from the quantum mechanical interference of the scattering of a photoelectron by the potential of the surrounding atoms. A photoelectron emitted by the photo-absorbing atom propagates as a spherical wave and spreads out over the solid. The amplitude of all the reflected electron waves adds either constructively or destructively to the spectrum of the absorbing atom as shown in Figure 5(a) and 5(b) respectively, and hence the X-ray absorption coefficient exhibits a typical oscillation. A crucial issue is the recognition that the photoelectron is not infinitely long lived; it decays as a function of time and distance, and thus the EXAFS cannot probe long-range distances. EXAFS can give only local structural information of about several angstroms around the selected atomic species.

Figure 5. Constructive and destructive interference of electron wave during and XAS event.

The recent availability of high-brightness synchrotron radiation sources has resulted in a prosperous development of XAS technique, and it is finding wide application area including the energy storage material field.
