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Introduction to X-Ray Absorption Spectroscopy

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Introduction to X-Ray Absorption Spectroscopy Introduction to X-ray Absorption Spectroscopy Bruce Ravel Synchrotron Science Group National Institute of Standards and Technology & Beamline for Materials Measurements National Synchrotron Light Source II MSAE E8235x Columbia University September 30, 2015 N Introduction to X-ray Absorption Spectroscopy 1 / 63 This Talk This is a long talk. We’ll take a break in the middle. http://waynocartoons.blogspot.com/2011 05 01 archive.html Cartoon c Wayno and Dan Piraro N Introduction to X-ray Absorption Spectroscopy 2 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary Part 1 The basic physics and chemistry of X-ray Absorption N Introduction to X-ray Absorption Spectroscopy 3 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary This Talk This talk is an introduction to the inner-shell spectroscopies, XAS and XRF. Outline An overview of the basic physics of inner shell spectroscopies An introduction to XAS and XRF beamline instrumentation A flavor of the sorts of science that can be accomplished with XAS and XRF, including examples from my own research and my beamline. My hope is that you will leave with a sense of how XAS and XRF might be applied to your research. N Introduction to X-ray Absorption Spectroscopy 4 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and XRF X-rayAbsorptionSpectroscopy and X-RayFluorescence spectroscopy These are inner shell spectroscopies. Inner shell means that an x-ray Spectroscopy means that some interacts primarily with a deep-core aspect of the interaction changes as electron rather than with a valence a function of photon energy. electron. N Introduction to X-ray Absorption Spectroscopy 5 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and XRF X-rayAbsorptionSpectroscopy and X-RayFluorescence spectroscopy These are inner shell spectroscopies. Inner shell means that an x-ray Spectroscopy means that some interacts primarily with a deep-core aspect of the interaction changes as electron rather than with a valence a function of photon energy. electron. N Introduction to X-ray Absorption Spectroscopy 5 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and XRF X-rayAbsorptionSpectroscopy and X-RayFluorescence spectroscopy These are inner shell spectroscopies. Inner shell means that an x-ray Spectroscopy means that some interacts primarily with a deep-core aspect of the interaction changes as electron rather than with a valence a function of photon energy. electron. N Introduction to X-ray Absorption Spectroscopy 5 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary The basic physical process in XAS and XRF n=4 N edges n=3 M edges X - ra n=2 L edges y in n=1 K edge 1 2 An incoming photon interacts with a deep-core electron. Shown here, a 1s electron is excited for a K-edge spectrum. 3 The deep-core electron is promoted to some unoccupied state above the Fermi energy, propagates away, and leaves behind a core-hole. 4 A short time later (1 or 2 femtoseconds), a higher-lying electron decays into the core-hole and emits a photon. Alternately, the energy from the higher-lying electron can be used to emit N an Auger electron. Introduction to X-ray Absorption Spectroscopy 6 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary The basic physical process in XAS and XRF n=4 N edges n=3 M edges n=2 L edges n=1 K edge 1 2 An incoming photon interacts with a deep-core electron. Shown here, a 1s electron is excited for a K-edge spectrum. 3 The deep-core electron is promoted to some unoccupied state above the Fermi energy, propagates away, and leaves behind a core-hole. 4 A short time later (1 or 2 femtoseconds), a higher-lying electron decays into the core-hole and emits a photon. Alternately, the energy from the higher-lying electron can be used to emit N an Auger electron. Introduction to X-ray Absorption Spectroscopy 6 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary The basic physical process in XAS and XRF X -r ay n=4 N edges ou t n=3 M edges n=2 L edges n=1 K edge 1 2 An incoming photon interacts with a deep-core electron. Shown here, a 1s electron is excited for a K-edge spectrum. 3 The deep-core electron is promoted to some unoccupied state above the Fermi energy, propagates away, and leaves behind a core-hole. 4 A short time later (1 or 2 femtoseconds), a higher-lying electron decays into the core-hole and emits a photon. Alternately, the energy from the higher-lying electron can be used to emit N an Auger electron. Introduction to X-ray Absorption Spectroscopy 6 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary The basic physical process in XAS and XRF A ug n=4 N edges er e - o n=3 M edges ut n=2 L edges n=1 K edge 1 2 An incoming photon interacts with a deep-core electron. Shown here, a 1s electron is excited for a K-edge spectrum. 3 The deep-core electron is promoted to some unoccupied state above the Fermi energy, propagates away, and leaves behind a core-hole. 4 A short time later (1 or 2 femtoseconds), a higher-lying electron decays into the core-hole and emits a photon. Alternately, the energy from the higher-lying electron can be used to emit N an Auger electron. Introduction to X-ray Absorption Spectroscopy 6 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary Elements and Beamlines K- or L-edges measured at a soft-X-ray beamline K-edges measured at a hard-X-ray beamline L-edges measured at a hard-X-ray beamline N Introduction to X-ray Absorption Spectroscopy 7 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary Characteristic energies Each element has a characteristic set of excitation and fluorescence energies. Iron: Z=26 α1 Edge Energy (eV) Lineα2 Transition Energy (eV) Strength K 7112 Kβ1 K-L3 6405.2 0.580 L3 706.8 Kβ3 K-L2 6392.1 0.294 L2 719.9 Kβ5 K-M3 7059.3 0.082 L1 844.6 K K-M2 7059.3 0.043 K K-M4,5 7110.0 0.001 Uranium: Z=92 α1 Edge Energy Lineα2 Transition Energy Strength K 115606 Lβ2 L3-M5 13614.0 0.686 L3 17166 Lβ5 L3-M4 13438.0 0.077 L2 20948 Lβ6 L3-N4,5 16387.7 0.181 L1 21757 L` L3-O4,5 17063.2 0.038 L L3-N1 15727.0 0.013 N Introduction to X-ray Absorption Spectroscopy L L3-M1 11618.0 0.005 8 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary A simple picture of X-ray absorption E E0 An(E − incidentE0) x-ray of energy is absorbed, destroying a core electron of binding energy and emitting a photo-electron with kinetic energy . The core state is eventually filled, ejecting a fluorescent x-ray or an Auger electron. An empty final state is required. No available state, no absorption! When the incident x-ray energy is larger than the binding energy, there is a sharp increase in absorption. µ(E) For an isolated atom, has a sharp step at the core-level binding N Introduction to X-ray Absorption Spectroscopy energy and is a smooth function of energy above the edge. 9 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary X-ray absorption in condensed matter R λ The ejected photo-electron can scatter from neighboring atoms. has some relationship to and there is a phase shift associated with the scattering event. Thus the outgoing and scattered waves interfere. The scattering of the photo-electron waveµ(E) function interferes with itself. (E − E0) depends on the density of states with energy at the absorbing atom. µ(E) This interference at the absorbing atom will vary with energy, causing N the oscillations in . Introduction to X-ray Absorption Spectroscopy 9 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and Valence State 0! 2+ ! 3+ ! 4+ As the valence increases Mn Mn Mn Mn the edge position shifts to higher energy. XAS is a direct measure of valence state Since each element has its own edge energy, an element’s valence can be measured even in a heterogeneous sample Since x-rays are deeply penetrating into matter, samples often require only preparation ··· Nowhatever assumption of symmetry or periodicity is made, so the sample can be crystalline, amorphous, thin film, in solution, surface sorbed, , N Introduction to X-ray Absorption Spectroscopy 10 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and Local Atomic Structure 0 2+ 3+ 4+ The different Mn species display big differences in the fine structure beyond the edge as the valence increases (Mn , Mn , Mn , Mn ). The white line and subsequent oscillations are quite different. The oscillatory portion of the spectrum can be isolated and ... ... Fourier transformed. This FT function can be 2+ interpreted to yield partial pair distribution 3+ 4+ functions of atoms about the absorber. The Mn-O distances are different for the Mn , Mn , and Mn and clearly different from the Mn-Mn distance in Mn metal. N Introduction to X-ray Absorption Spectroscopy 11 / 63 X-rays and electrons XAS XRF XAS measurement Vocabulary XAS and Local Atomic Structure 0 2+ 3+ 4+ The different Mn species display big differences in the fine structure beyond the edge as the valence increases (Mn , Mn , Mn , Mn ).
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