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Extended X-ray absorption fine structure
About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.
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TL;DR: In this paper, the absorption of free carriers in $n$-type silicon has been studied in a spectral region from 1 to 45 microns for samples of various impurity and carrier concentrations.
Abstract: The absorption by free carriers in $n$-type silicon has been studied in a spectral region from 1 to 45 microns for samples of various impurity and carrier concentrations. In all samples measured the absorption consists of a band between 1.5 and 5 microns in addition to an absorption which rises smoothly with wavelength. The absorption band is found to be proportional to the carrier concentration in samples doped with different donor impurities. It is suggested that the band is associated with the excitation of carriers to a higher lying energy band. The behavior of the smoothly rising portion of the absorption curve is in agreement with the Fan and Fr\"ohlich theory of free-carrier absorption. Quantitative agreement is obtained with a value of $0.3m$ for the effective mass.
264 citations
TL;DR: In this article, the shape of collision-broadened absorption lines is extended to high power levels of the exciting radiation, and transitions among the molecular states are then induced at a rate that is not negligible compared with the collision rate, thus invalidating the assumption of thermal equilibrium.
Abstract: The investigation of Van Vleck and Weisskopf, on the shape of collision-broadened absorption lines, is extended to high power levels of the exciting radiation. Transitions among the molecular states are then induced at a rate that is not negligible compared with the collision rate, thus invalidating the assumption of thermal equilibrium. The theory is based on a quantum transcription of the previous semiclassical treatment, the essential tool being the density matrix. When the resonant frequencies of the molecule are widely spaced, an absorption line is ultimately broadened as the power level increases. Correspondingly, the peak absorption coefficient decreases, the power absorbed per unit volume approaching saturation with increasing incident power. It is shown that the broadening of an absorption line is not to be attributed to any intrinsic modification of the line shape, but rather to a frequency dependent alteration of the energy level populations. A molecule for which all resonant frequencies coincide, a one-dimensional harmonic oscillator, is treated in Appendix II. No saturation effect occurs here, since the absorption is independent of the molecular distribution among the oscillator energy levels.
263 citations
TL;DR: In this article, the authors used synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy to investigate the reduction of aqueous Cr(VI) to Cr(III) in magnetite-bearing soils from Cr-contaminated sites.
263 citations
TL;DR: There has been substantial progress in recent years, both in the theory and in ab initio codes for calculations of X-ray absorption spectra (XAS), and in particular, the near edge structure (XANES) as discussed by the authors.
262 citations
TL;DR: In this paper, the properties of the compound NiCo2O4 with spinel-related structure have been examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS), and the results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo 2O4 of {Ni3+0.
Abstract: The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and
Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.
261 citations