Synchrotron X-ray Absorption Spectroscopy - Stanford Synchrotron ...

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Nomenclature There are a large number of names and acronyms in use – they all refer to the same thing or are closely related… Edge Spectra Near-Edge Spectra Near-Edge X-ray Absorption Fine Structure (NEXAFS) X-ray Absorption Near-Edge Structure (XANES) I. J. Pickering and G. N. George GEOL 498.3/898.3 Nomenclature Sometimes, but not always, “XANES” is used to refer to the region just above the edge, which is more readily calculable using multiple scattering theory. near-edge } } XANES EXAFS I. J. Pickering and G. N. George GEOL 498.3/898.3 Nomenclature What is a “White Line”? The term “white line” refers to an intense absorption in the near-edge. The nomenclature dates from the days when spectra were recorded on strips of photographic film, and such intense absorption peaks showed up as a heavily exposed line on the developed film. White Line White Line photographic film spectrum I. J. Pickering and G. N. George GEOL 498.3/898.3
X-ray absorption near-edge spectra Intense features arise due excitation of transitions from the core level to vacant levels, close to the highest occupied molecular orbital. hν 2s, l=0 1s, l=0 vacant orbital 2p, l=1 nucleus electron electron-hole I. J. Pickering and G. N. George GEOL 498.3/898.3 Core level What is a near-edge spectrum The photoelectron is excited to a variety of bound states lying below the threshold energy. Transitions to bound states observed spectrum Threshold, E 0 I. J. Pickering and G. N. George GEOL 498.3/898.3 µ ( ) = ∑ ψ i H Near-edge spectra X-ray absorption is given by Fermi’s Golden Rule: E ψ f 2 ψ i - the initial state wavefunction ψ f - the final state wavefunction H -the interaction If we wish to quantify spectra, we have two alternatives – evaluate the integral as completely as possible (molecular orbital approach) or use multiple scattering theory. Molecular orbital approach. A chemistry perspective – the X-ray excites transitions between the core level and a molecular orbital. Quantification is non-trivial, but this approach is highly successful in understanding spectra. Multiple scattering approach. A physics perspective – the X-ray excites a lowenergy photo-electron which undergoes extensive multiple scattering by nearby atoms. This success of this approach is limited (to date). It usually cannot model features due to low-lying bound-state transitions. I. J. Pickering and G. N. George GEOL 498.3/898.3
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