Computational X‐Ray Spectroscopy

Carravetta, Vincenzo; Ågren, Hans

doi:10.1002/9781118008720.ch3

Cited by 9 publications

(13 citation statements)

References 223 publications

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“…The complex polarization propagator (CPP) method [36][37][38][39][40][41] is similar to time-dependent methods, but explicitly accounts for the linewidth of the peaks via an imaginary damping factor which, when graphed, generates the spectrum. At the Hartree-Fock level, the static exchange approximation (STEX) [42][43][44] models core excitations as an electron being acted upon by the core ionized molecule, the latter of which is optimized separately in order to give a complete account of the orbital relaxation in the molecule.…”

Section: Introductionmentioning

confidence: 99%

4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂

South

Shee

Mukherjee

et al. 2016

Phys. Chem. Chem. Phys.

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We present a 4-component relativistic study of uranium 2p3/2 ionization and excitation in the isoelectronic series UO2(2+), OUN(+) and UN2. We calculate ionization energies by ΔSCF at the Hartree-Fock (HF) and Kohn-Sham (KS) level of theory. At the ΔHF level we observe a perfectly linear chemical shift of ionization energies with respect to uranium atomic charges obtained from projection analysis. We have also developed a non-canonical 2nd-order Møller-Plesset code for wave function based correlation studies. We observe the well-known failure of Koopmans' theorem for core ionization due to the dominance of orbital relaxation over electron correlation effects. More unexpectedly, we find that the correlation contribution has the same sign as the relaxation contribution and show that this is due to a strong coupling of relaxation and correlation. We simulate uranium L3 XANES spectra, dominated by 2p3/2 → U6d transitions, by restricted excitation window time-dependent density functional theory (REW-TDDFT) and the complex polarization propagator (CPP) approach and demonstrate that they give identical spectra when the same Lorentz broadening is chosen. We also simulate XANES spectra by the Hartree-Fock based static exchange (STEX) method and show how STEX excitation energies can be reproduced by time-dependent Hartree-Fock calculations within the Tamm-Dancoff approximation. We furthermore show that Koopmans' theorem provide a correct approximation of ionization energies in the linear response regime and use this observation to align REW-TDDFT and CPP spectra with STEX ones. We point out that the STEX method affords the most detailed assignment of spectra since it employs virtual orbitals optimized for the selected core ionization. The calculated XANES spectra reflect the loss of bound virtual orbitals as the molecular charge is reduced along the isoelectronic series.

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Section: Introductionmentioning

confidence: 99%

4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂

South

Shee

Mukherjee

et al. 2016

Phys. Chem. Chem. Phys.

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“…This requires either the comparison with spectra measured for model compounds or with theoretical predictions. To be able to extract additional information from X-ray spectroscopic measurements, the development of theoretical methods for the calculation of X-ray spectra is essential [19].…”

Section: Introductionmentioning

confidence: 99%

“…In contrast, for describing prepeaks and the XANES region in XAS spectroscopy and for predicting XES spectra, quantum-chemical approaches are usually required. To this end, a wide range of quantum-chemical methods have been developed for describing excitations from core orbitals (for reviews, see, e.g., Refs [19,22,23]). Widely used are the static-exchange approximation (STEX) [24][25][26], approaches based on tran-sition potential density-functional theory (DFT) [27,28], and time-dependent densityfunctional theory (TD-DFT) [23].…”

Section: Introductionmentioning

confidence: 99%

Origin-independent calculation of quadrupole intensities in X-ray spectroscopy

Bernadotte

Atkins

Jacob

2012

The Journal of Chemical Physics

147

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For electronic excitations in the ultraviolet and visible range of the electromagnetic spectrum, the intensities are usually calculated within the dipole approximation, which assumes that the oscillating electric field is constant over the length scale of the transition. For the short wavelengths used in hard X-ray spectroscopy, the dipole approximation may not be adequate. In particular, for metal K-edge X-ray absorption spectroscopy (XAS), it becomes necessary to include higher-order contributions. In quantum-chemical approaches to X-ray spectroscopy, these socalled quadrupole intensities have so far been calculated by including contributions depending on the square of the electric-quadrupole and magnetic-dipole transition moments. However, the resulting quadrupole intensities depend on the choice of the origin of the coordinate system. Here, we show that for obtaining an originindependent theory, one has to include all contributions that are of the same order in the wave vector consistently. This leads to two additional contributions depending on products of the electric-dipole andelectric-octupole and of the electric-dipole and magnetic-quadrupole transition moments, respectively. We have implemented such an origin-independent calculation of quadrupole intensities in XAS within timedependent density-functional theory, and demonstrate its usefulness for the calculation of metal and ligand K-edge XAS spectra of transition metal complexes.2

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“…In the past, the description of shake-up effects in core-hole photoemission spectra has been investigated in the framework of quantum-chemical calculations 8 , by using approaches based on model hamiltonians 1,9,10 or first-principles modified approaches 11 . The occurrence of these effects in XPS is well established but they have also been shown to occur in M 4,5 edges of mixed-valent compounds 12 and in L 2,3 X-ray absorption spectra (XAS) of transition metals and rare earths compounds 13,14 and were proposed as a possible explanation of the double peak structure in dipolar K-edge XAS of high T c cuprates 15 and copper compounds in general 16 .…”

Section: Introductionmentioning

confidence: 99%

K-edge x-ray absorption spectra in transition-metal oxides beyond the single-particle approximation: Shake-up many-body effects

et al. 2012

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The near edge structure (XANES) in K-edge X-ray absorption spectroscopy (XAS) is a widely used tool for studying electronic and local structure in materials. The precise interpretation of these spectra with the help of calculations is hence of prime importance, especially for the study of correlated materials which have a complicated electronic structure per se. The single particle approach, for example, has generally limited itself to the dominant dipolar cross-section. It has long been known however that effects beyond this approach should be taken into account, both due to the inadequacy of such calculations when compared to experiment and the presence of shake-up many-body satellites in core-level photoemission spectra of correlated materials. This effect should manifest itself in XANES spectra and the question is firstly how to account for it theoretically and secondly how to verify it experimentally. By using state-of-the-art first principles electronic structure calculations and 1s photoemission measurements we demonstrate that shake-up manybody effects are present in K-edge XAS dipolar spectra of NiO, CoO and CuO at all energy scales. We show that shake-up effects can be included in K-edge XAS spectra in a simple way by convoluting the single-particle first-principles calculations including core-hole effects with the 1s photoemission spectra. We thus describe all features appearing in the XAS dipolar cross-section of NiO and CoO and obtain a dramatic improvement with respect to the single-particle calculation in CuO. These materials being prototype correlated magnetic oxides, our work points to the presence of shake-up effects in K-edge XANES of most correlated transition metal compounds and shows how to account for them, paving the way to a precise understanding of their electronic structure.

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Computational X‐Ray Spectroscopy

Cited by 9 publications

References 223 publications

4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂

4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂

Origin-independent calculation of quadrupole intensities in X-ray spectroscopy

K-edge x-ray absorption spectra in transition-metal oxides beyond the single-particle approximation: Shake-up many-body effects

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Computational X‐Ray Spectroscopy

Cited by 9 publications

References 223 publications

4-Component relativistic calculations of L3ionization and excitations for the isoelectronic species UO22+, OUN+and UN2

4-Component relativistic calculations of L3ionization and excitations for the isoelectronic species UO22+, OUN+and UN2

Origin-independent calculation of quadrupole intensities in X-ray spectroscopy

K-edge x-ray absorption spectra in transition-metal oxides beyond the single-particle approximation: Shake-up many-body effects

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4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂

4-Component relativistic calculations of L₃ionization and excitations for the isoelectronic species UO₂²⁺, OUN⁺and UN₂