Introduction

X-ray absorption near edge structure (XANES) may also be referred to as near edge X-ray absorption fine structure (NEXAFS) is one of the X-ray absorption spectroscopy (XAS) methods which measures the photoabsorption cross-section associated to excitation or emission of a core level electron.^{[1, p.168-167]} The method is based on the specificity of the elements and provides a wide variety of chemical information on the sample. Therefore the purpose of the main analysis is difficult to describe simply. The special ability of XANES is to study the nature of the individual atoms of the deeper layers of the sample, in addition to the surface layers. ^{[2, p.361-381]}

Overview

XANES can be used to determine oxidation states, molecular orbitals, band structure and coordination environment (e.g. octahedral, tetrahedral structures). To collect this data, the XANES method includes single and multiple events from the first atomic shell surrounding the center atom, as well as multiple layers events from multiple distant atomic shells and from neighbor atoms. ^{[2, p.361-381][3, p.160-161,174-178]} Figure 1 shows the scattering of the photoelectron (blue sphere) from the central atom to the neighboring atoms in XANES.

Figure 1. Picture of photoelectron scattering processes in the XANES multiple scattering regime. Figure: Erin Makara.

XANES uses an energy range of 0 to 50 eV. In this range, very sensitive information from the sample is obtained, such as determined from a sample the radial distances between the absorbing atom and two or three shells of atoms around it. In addition the number of atoms in these shells and the temperature factors can be determined.^{[4, p.61,76]}Each element has a special XANES spectrum which can be used also, as fingerprints to identify different elements. ^{[1, p.167-168]} The Figure 2 shows the XANES energy range and a very simplified peak.

Figure 2. XANES and EXAFS energy areas. Figure from Wikipedia. License: CC BY-SA 3.0.

Working principles

Photo absorption

XANES spectroscopy measures the excitation or emission of a core level electrons caused by X-ray photon absorption. The characteristics of the XANES spectrum are related to the transfer of the core electron or more electrons populate unoccupied bound or continuum states. ^{[1, p.167-168]} This can be illustrated by the one-particle theory (Figure 3), where the incoming X-ray photon from a particular energy range (E=hv) transfers its energy to an electron in the core of the atom. Photon absorption causes electron emission and formed photoelectron, leaving a hole in the atomic core level. The core hole can be filled in two ways, forming different characterizing rays that are specific to each element. As the electron in the shell of a higher energy level fills the hole, a fluorescent photon is emitted. In the Auger phenomenon, the electron in the shell of the higher energy level fills the hole that causes the adjacent or nearby electron to emit, forming the Auger electron. ^{[2, p.361-381]} The XANES measurement results are the sum of the possible final states of these photoelectrons which can also be a bound state such as an exciton. ^{[1, p.167-168]}

Figure 3. Photoelectron formation and consequent fluorescence photon or Auger electron formation. Figure: Jenni Ahlstedt.

Absorption edge

XANES measures the so-called edge which is the occurrence of the above absorption phenomena at specific energy values. The edges consist of electron shells (K, L, M, N, O) and atomic quantum numbers (n = 1, 2, 3, 4, 5). Each element has a specific core binding energy which produce an absorption edge specific of the element. ^{[4, p.61,76]} The Figure 4 shows the K, L, and M edges and their quantum numbers and orbitals. In addition, the figure illustrates the energy regions of the edges relative to each other.

Figure 4. The edges of K, L, M and their quantum numbers and orbitals. Figure from Wikipedia. License: CC BY-SA 3.0.

Experimental method

Principle of operation of XAS is as follows (Figure 5). The monochrome photon beam sourced by the synchrotron ring is guided through the sample. Detectors measure changes in the intensity of the incoming (I_o) and transmitted (I₁) beams, the absorption coefficient of the sample. The energy dependence of the absorption coefficient is collected by a stepwise scan of the photon energy in the monochromatic beam with the Bragg monochromator. ^[5]

Figure 5. Simple schematic of X-ray absorption spectroscopy (XAS) with synchrotron radiation sources. Figure: Jenni Ahlstedt.

The exponential attenuation of X-rays in a homogeneous medium, the absorption coefficient (µ) for a given photon energy (E) is given by the formula below, where d is the sample thickness: ^[5]

An example of analyzing results

As stated previously, XANES can be used to determine elements and a wide variety of chemical information of sample. The peak shapes of the XANES spectra are formed by edges. Their analysis can be divided into three parts (Figure 6). Before the actual edge peak, there is a pre-edge (pink region in the Figure 6) whose intensity is affected by the coordination geometry of the central atom. The actual edge (green region in the Figure 6) position change is commonly used to determine the valence state, but the oxidation state can also be qualitatively determined. The near-edge region (light blue region in the Figure 6) occurring after the actual edge indicate coordination shells when the emitted photoelectron scatters off neighboring atoms. ^{[2, p.361-381]}

Figure 6. A description of specific parts of the XANES spectrum. Figure: Jenni Ahlstedt.

Advantages and Disavantages

An advantage of the XANES measurement method is its sensitivity to the multi-electronic phenomenon, which provides additional information on the material under investigation and allows the extraction of a three dimensional structure from its XANES spectrum. The disadvantage is that X-rays can be destructive to the sample and thus cause damage to the sample during the test. Disadvantages may also include some accuracy problems with the XANES measurement method and a particular sensitivity to the effects of core hole excitons, which can only be treated with approximately. ^{[4, p.61,76]}

References