Oxygen K-edge X-ray Absorption Spectra

Abstract

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Figure 1

Overview of the detection techniques of X-ray absorption spectroscopy, including transmission, electron yield, fluorescence yield, and ion yield methods. Sample measurement details are given in green and the pressure in blue.

Figure 2

Oxygen K-edge of a mixture of methanol and oxygen at 0.52 mbar that reacts on a Cu foil at 520 K. The detectors near the surface detect the signal from the gas phase and the surface; the detector far from the surface detects the pure gas phase spectrum.

Figure 3

(Left) XAS final-state approximations: (a) Z+1 equiv core–hole, (b) full core–hole, (c) excited core–hole, (d) Slater transition state, (e) transition state approximation. (Right) The FCH, XCH, and HCH approximations applied to liquid H₂O.

Figure 4

Representation of pseudo-wavefunction and pseudopotential. The pseudo-wavefunctions are smoother than the real wave function near the nuclei (where r_c is the core radius), so the number of plane waves required is reduced.

Figure 5

(Left) Beryllium K-edge of BeO calculated with and without core–hole. (Left) Oxygen K-edge of BeO calculated with and without core–hole.

Figure 6

(a) Theoretical GeO₂ oxygen K-edge with and without core–hole compared with experiment. (b) Theoretical oxygen K-edge XAS without core–hole compared with a ground-state oxygen DOS calculation.

Figure 7

Oxygen K-edge spectra can in the first approximation be calculated from DFT codes, where molecules are usually calculated from molecular DFT codes and solids with band structure codes or multiple scattering. The oxygen p-contribution to the MOs or the oxygen p-projected DOS can be compared with the oxygen K-edge spectral shape, where matrix elements are also included. As a next step, the core–hole effect can be included, where many different procedures have been used both for molecules and for solids. In the case of solids, one has to perform a supercell calculation to prevent the core–hole from interacting with each other. Formally, the correct way to calculate the oxygen K-edges is to apply electron–hole excitation schemes such as TD-DFT or BSE, where the complexity of the calculations necessitates approximations. In systems where the dynamics of the system is important (including liquids), the calculations must be combined with molecular dynamics calculations. For example, a series of atomic positions can be determined and their X-ray absorption spectra added. Because of the 300 meV lifetime broadening, (in most cases) the effects of multiplets, orbital polarization, and magnetic exchange are not visible. Vibrations are visible in the case of (small) molecules.

Figure 8

Experimental atomic oxygen K-edge X-ray absorption spectrum from ref (145). The 1s to 2p transition is visible at 527 eV. The Rydberg states are visible above 540 eV.

Figure 9

Molecular orbital diagram of CO, NO, and O₂. The three MO energy schemes are equivalent, where in the case of O₂, the order of the bonding σ_2p and π_2p states is inverted due to a decreased influence of the 2s states. The antibonding orbital π_2p^* is filled with, respectively, 0, 1, and 2 electrons, creating the triplet ground state for O₂.

Figure 10

(a) Oxygen K-edge of O₂ from ref (158) including a high-resolution inset of the Rydberg states. (b) Energy level diagram and allowed transitions, including the ionization potential for spin up and down indicated as ²Σ and ⁴Σ. The exchange splitting arises from differences in the magnetic exchange interactions in the final states yielding the experimental peaks B and C.

Figure 11

Oxygen K-edge spectrum of NO where the ionization limits are indicated as ¹Π and ³Π. The three theoretical components (²Σ^–, ²Δ, and ²Σ⁺) in the 1s2p peak at 532.7 eV are indicated.

Figure 12

Oxygen K-edge spectrum of CO where the ionization potential (IP) is indicated.

Figure 13

Molecular orbital diagrams of CO₂, NO₂, and O₃. The MO schemes are equivalent to the difference that the number of electrons in the 2pπ* orbital is respectively 0, 1, and 2.

Figure 14

Oxygen K-edge TIY spectrum of ozone. The peaks at 529 and 536 eV are excitations into the same 2pπ* state, from, respectively, the terminal O_T and the center oxygens O_C. Note that the small peak at 531 eV is due to the presence of some O₂.

Figure 15

Oxygen K-edge spectrum of CO₂ from ref (191). The 2pπ* state is seen at 535 eV. Above 538 eV, the 2pσ* and the Rydberg states are visible.

Figure 16

Oxygen K-edge of oxygen adsorbed on a Cu(100) surface (points) compared with the Cu L₃-edge shifted by 401 eV.

Figure 17

Oxygen K-edge of CO on Ni(100). The 2π* orbital is visible at 534.0 eV and the σ* orbital at 550.0 eV.

Figure 18

X-ray absorption spectra of gas-phase water, liquid water at 299 K, and ice grown on BaF₂ at 144 K.

Figure 19

(a) Time evolution of the oxygen K-edge spectra of cysteine at time zero and after 17 min without any rescaling. (b) Oxygen K-edge of DPPC at grazing incidence (GI) and normal incidence (NI) before and after the irradiation.

Figure 20

Experimental oxygen K-edge spectra of the 22 amino acids and their molecular formulas. In the table, the peak positions together with their assignments are reported.

Figure 21

Oxygen K-edge spectra of 0.6 M glycine (aq) as a function of pH. A small red shift is observed for the acidic solution, which has been illustrated by a dotted line through the center of each peak. This shift is caused by the protonation of the carboxylate subgroup at low pH, and the resultant breaking of the degeneracy in the O 1s → π_CO^* transition.

Figure 22

(a) Oxygen K-edge spectra of uracil, thymine, cytosine, and guanine together with their molecular structure and the peak assignment. (b) Effect on the oxygen K-edge spectra from adding sugar and phosphate groups of DNA to thymine.

Figure 23

Oxygen K-edge spectra of the oxide Li₂O, the peroxide Li₂O₂, and the superoxides LiO₂, CsO₂, and KO₂ compared to O₂. The σ and π character of the antibonding states are indicated.

Figure 24

Oxygen K-edge spectra of alkaline earth oxides showing discrepancies between the experiments: MgO,^,,, CaO,⁻ SrO,^,, and BaO.^,,

Figure 25

(a) Oxygen K-edge spectra of SiO₂ and GeO₂ in the quartz structure (b) and (c) oxygen K-edge spectra of SiO₂, GeO₂, and SnO₂ in the rutile structure (d). The natures of the final states are assigned based on the DOS calculations.

Figure 26

Oxygen K-edge spectra of amorphous B₂O₃ at 1 bar and 8.4 GPa, LiBO₂, Li₂B₄O₇, and Li₃BO₃, with the fraction of BO₃ units and nonbridging oxygens indicated. Nonbridging orbitals induce a shift of the π* feature to lower energy and the conversion from BO₃ to BO₄ induces the decrease of its intensity.

Figure 27

Oxygen K-edge spectra of carbonates: BaCO₃, Li₂CO₃, CaCO₃, and Na₂CO₃, all showing the π* antibonding state from the C=O bonds of the CO₃^–2 units.

Figure 28

Oxygen K-edge spectra of ZnO (wurtzite) compared to Fe:ZnO (red), CdO (rock salt), and HgO (orthorhombic, montroydite). Similarities in the spectrum shape and assignment are highlighted despite the differences in the crystal structures.

Figure 29

Oxygen K-edge spectra of (a) 5s⁰ oxides (In₂O₃, SnO₂, Sb₂O₅) and (b) 5s² oxides (SnO, Sb₂O₃, TeO₂). Data are reproduced from ref (234) for all but TeO₂. The cation contributions to the oxygen 2sp antibonding states are given according to the p-DOS calculations from McLeod et al.

Figure 30

Oxygen K-edge spectra of BaBiO₃ and the BaPbO₃.

Figure 31

Interpretation of the oxygen K-edge XAS spectrum of a 3d transition metal oxide. The oxygen 1s core state is given in green at 530 eV binding energy. The occupied oxygen 2s and 2p bands are given as a combination of oxygen (green) and metal (red). The empty states are given with striped colors: The ratio of t_2g and e_g states is 6:4, but the oxygen contribution (in green) is equivalent. At higher energy, the metal 4sp band is given. The experimental spectrum of TiO₂ is given as example.

Figure 32

Intensity of the t_2g and e_g peaks of a 3d⁰-system due to degeneracy, number of overlaps, and overlap strengths.

Figure 33

Intensity of the t_2g and e_g peaks of a 3d³-system due to the exchange splitting, degeneracy, the number of overlaps, and the overlap strengths.

Figure 34

Oxygen K-edge of the 3d monoxides VO, MnO, FeO, CoO, and NiO: below 535 eV, the 3d band related peaks; and from 536 to 543 eV, the metal 4sp band.

Figure 35

Oxygen K-edge of the corundum oxides Ti₂O₃, V₂O₃, Cr₂O₃, Mn₂O₃, Fe₂O₃, and Co₂O₃: Below 535 eV, the 3d band related peaks; and from 536 to 545 eV, the metal 4sp band.

Figure 36

Oxygen K-edge of the rutile oxides TiO₂, VO₂, CrO₂, and MnO₂: Below 535 eV, the 3d band related peaks; and from 536 to 547 eV, the metal 4sp band.

Figure 37

(a) Crystal structure of cubic perovskite with the ABO₃ formula. (b) Crystal structure of layered perovskite with the A₂BO₄ formula. A: green ; B: orange, O: red.

Figure 38

Oxygen K-edge spectra of LaMO₃ perovskite systems for the transition metals Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. (H.S.: high spin; L.S.: low spin).

Figure 39

Oxygen K-edge spectra of the oxyanions MO₄^n– with M = Cr, Mo, W, Mn, Tc, and Re.

Figure 40

Oxygen K-edge of LaTiO₃ and SrTiO₃ perovskites. The titanium 3d, lanthanum 5d/strontium 4d, and titanium 4sp related peaks are indicated.

Figure 41

Oxygen K-edge of Cr₂O₃. (a) ref (339); (b) ref (302); (c) ref (340); (d) ref (341) There is variation visible in the spectral shape and especially in the energy calibration.

Figure 42

Oxygen K-edge of LaMnO₃ and SrMnO₃. The peaks are caused by the Mn 3d states, the La and Sr 5d/4d states, and the Mn 4sp states, as indicated.

Figure 43

Oxygen K-edge of MnO, LiMnO₂, LiMn₂O₄, Li₂MnO₃. The 3d⁵ oxide MnO contains impurities from Mn³⁺ visible in the intensity before 529 eV. The 3d⁴ oxide LiMnO₂ has a broad 3d part due to a combination of the octahedral crystal field, exchange, and the Jahn–Teller distortion. The 3d³ oxide Li₂MnO₃ has a first peak related to a combination of spin-up e_g and spin-down t_2g states. LiMn₂O₄ is a mixed valence Mn³⁺/Mn⁴⁺ oxide, and in the first approximation its 3d part is a combination of the spectra of Mn³⁺ and Mn⁴⁺ sites.

Figure 44

Oxygen K-edge of LaFeO₃ and SrFeO₃ perovskite. The peaks are, respectively, caused by the Fe 3d states, the La and Sr 5d/4d states, and the Fe 4sp states, as indicated.

Figure 45

Oxygen K-edge spectra of Y₂O₃, tetragonal, and monoclinic (m-) ZrO₂ and cubic yttrium stabilized zirconia (c-YSZ), NbO₂ and Nb₂O₅, MoO₃ and MoO₂, and RuO₂. Visible are, respectively, the t_2g states, the e_g states, and the empty sp bands of the metal ion.

Figure 46

Oxygen K-edge of Sr₂RuO₄ layered perovskite with the incident electrical field parallel to the layer (ε_ab) and perpendicular to the layer (ε_c). The contributions from the apical and in-plane oxygens are highlighted.

Figure 47

Oxygen K-edge of HfO₂ compared to its 3d and 4d analogs TiO₂ (rutile) and ZrO₂ and to IrO2 (rutile).

Figure 48

Oxygen K-edge spectra of monoclinic WO₃ and cubic ReO₃ compared to MoO₃ (orthorhombic) with the assignment of the spectral features.

Figure 49

Oxygen K-edge spectra of Sr₂IrO₄ layered perovskite compared the SrIrO₃ perovskite with the incident electrical field parallel to the layer ε_ab and perpendicular to the layer ε_c.

Figure 50

(a) Oxygen K-edge spectra of the Ln₂O₃ oxides digitized from ref (87); (b) the corresponding qualitative energy band diagrams.

Figure 51

(a) Oxygen K-edge spectra of the cubic CeO₂, PrO₂, and TbO₂ with the corresponding band assignment in (b) cubic fluorite structure (oxygen atoms in red and lanthanide atoms in yellow) and (c) the corresponding schematic DOS adapted from ref (457).

Figure 52

Oxygen K-edge spectrum of cubic La₂O₃ and hexagonal La₂O₃.

Figure 53

Oxygen K-edge spectra of UO₂,NpO₂, and PuO₂.

Figure 54

Polarized oxygen K-edge spectrum of the uranyl UO₂²⁺ species in Cs₂UO₂Cl₄ single crystal. The σ and π characters of the probed states are revealed with the incident light polarization respectively parallel and perpendicular to the uranyl bond.