XAFS Classroom

Introduction: X-ray Absorption Fine Structure (XAFS) spectroscopy has been widely used in chemical catalysis, materials science and other fields, and has gradually become a conventional characterization method. Therefore, this official account will continue to (from time to time) to publish content about XAFS principles, tests, data analysis, and article appreciation, hoping to provide XAFS users with a place to learn the basic knowledge of XAFS.

XAFS, which we have been mentioning, is essentially the correlation between the absorption coefficient of X-rays μ (E) of a substance and the energy E of X-rays. In general, the absorption coefficient of a substance for X-rays μ (E) shows a trend of 1/E³, as shown in Figure 1, and the overall trend decreases as the energy of the photon increases. However, in 1913, de Broglie et al. first discovered that large absorption edges occur due to resonance at the absorption edges of specific elements, and in 1920, with the help of more sophisticated instruments, Hugo Fricke et al. discovered that the spectral lines after the absorption edges were not smooth, but had periodic oscillations, which were also defined as "fine structure". Since then, the door has been opened to XAFS research.

Fig.1. X-ray absorption cross-section of platinum

In the decades that followed, scientists focused on tracing the origin of these fine structures, and experienced the debate of long-range ordering and short-range ordering, until 1971, when Sayers, Stern, and Lytle pioneered the idea of extracting and separating the structural information of different shells through Fourier transforms, which led to an essential understanding of the basic principles of fine structures. In addition, with the construction and application of synchrotron radiation devices, XAFS has become a practical and fast analysis method.

Fig.2 K-edge XAFS spectra of Fe samples

Figure 2 shows the XAFS spectrum of the K absorption edge of Fe of an iron sample, which can be divided into three parts: the first part is the anterior edge part before -20 eV before the absorption edge, which mainly exists as a substrate, the second part is the X-ray absorption near-edge structure (XANES) in the range from -20 eV before the absorption edge to 30 eV after the absorption edge, which has a strong oscillation, and the third part is the X-ray absorption near-edge structure (XANES) in the range of -20 eV before the absorption edge, which is more intense, and the third part is the X-ray absorption near-edge structure (XANES) in the range of -20 eV before the absorption edge to 30 eV after the absorption edge, and the third part is the X-ray absorption near-edge structure (XANES) in the range of -20 eV before the absorption edge to 30 eV after the absorption edge, and the third part is the 30 after the absorption edge The oscillation after eV is less intense than that of the near-lateral part.