Resonant Soft X-ray Emission Spectra Research Articles

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This article shows our recent study on electronic structure of several typical ionic liquids. Ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy and soft x-ray emission spectroscopy (SXES) were used for elucidating the electronic structure of ionic liquids based on methylbutylimidazolium ([Cn mim]+) cation. Non-resonant SXES spectra measured above N, O, and F 1s edges selectively probed the partial density of states of the cation and anion. They give a clear evidence that the highest occupied molecular orbital of the [C4 mim]+ cation contributes to the topmost occupied states of the ionic liquids [C4 mim]+PF6−. Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C4 mim]+PF6− is solely determined by the [C4 mim]+ cation, in contrast to usual ionic crystals like NaCl and NaBr. The ionic liquids composed of fluoride anions PF6− and BF4− have anomalous electronic structure. Such basic knowledge about the electronic structure of ionic liquid must be helpful for new molecular design of ionic liquids.

In situremoval of carbon contamination from optics in a vacuum ultraviolet and soft X-ray undulator beamline using oxygen activated by zeroth-order synchrotron radiation

Carbon contamination of optics is a serious issue in all soft X-ray beamlines because it decreases the quality of experimental data, such as near-edge X-ray absorption fine structure, resonant photoemission and resonant soft X-ray emission spectra in the carbon K-edge region. Here an in situ method involving the use of oxygen activated by zeroth-order synchrotron radiation was used to clean the optics in a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A at the Photon Factory in Tsukuba, Japan. The carbon contamination of the optics was removed by exposing them to oxygen at a pressure of 10(-1)-10(-4) Pa for 17-20 h and simultaneously irradiating them with zeroth-order synchrotron radiation. After the cleaning, the decrease in the photon intensity in the carbon K-edge region reduced to 2-5%. The base pressure of the beamline recovered to 10(-7)-10(-8) Pa in one day without baking. The beamline can be used without additional commissioning.

Anomalous electronic structure of ionic liquids determined by soft x-ray emission spectroscopy: Contributions from the cations and anions to the occupied electronic structure

Soft x-ray emission spectroscopy was used for elucidating the electronic structure of ionic liquids [C(4)mim](+)PF(6)(-) and [C(4)mim](+)OTf(-), where [C(4)mim](+) stands for methylbutylimidazolium cation and OTf(-) for the trifluoromethanesulfonate anion. Nonresonant spectra measured above N, O, and F 1s edges selectively probed the molecular orbitals (MOs) of the cation and anions. They give a clear evidence that the highest occupied molecular orbital of the [C(4)mim](+) cation contributes to the topmost occupied states of the ionic liquids [C(4)mim](+)PF(6)(-), while both cationic and anionic MOs contribute for the case of [C(4)mim](+)OTf(-). Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C(4)mim](+)PF(6)(-) is solely determined by the [C(4)mim](+) cation, in contrast to usual ionic crystals. The ionic liquids form a new class of the ionic materials from the viewpoint of the electronic structure.

Li insertion into V6O13 battery cathodes studied by soft x-ray spectroscopies

Changes in the electronic structure of V6O13 on lithium-ion insertion into battery cathodes were studied by soft x-ray absorption (SXA) spectroscopy and resonant soft x-ray emission (SXE) spectroscopy. SXA and resonant SXE spectra were recorded ex situ for cycled battery cathodes discharged to different potentials corresponding closely to distinct lithiated stages (LixV6O13,x=0,1,…,6). Large systematic changes were observed in the vanadium and oxygen x-ray spectra, reflecting the effects of electrochemical reduction associated with the Li-ion insertion. Spectral shape analysis indicates that a large fraction of the vanadium ions have been reduced to V3+ ions for the highest degree of lithiation, x=6. Nevertheless, further lithiation may be possible, in view of the linear development of the vanadium and oxygen bands on charge uptake.

Polarization dependence in resonant soft X-ray emission spectroscopy of 3 d transition metal compounds

We report experimental studies of polarization dependence in resonant soft X-ray emission of 3 d transition metal compounds. In metal 2 p resonant soft X-ray emission spectra of 3 d 0 systems, transition to the bonding and antibonding states between 3 d 0 and 3d 1 L configurations shows polarization dependence. In TiO 2, the polarization dependence is drastic, whereas it is slightly weak in ScF 3 and extremely weak in KMnO 4. The results are explained by a local symmetry selection rule, strength of hybridization, and phase relation of wave-functions among initial, intermediate, and final states. In oxygen 1 s resonant excitation of undoped cuprate, Sr 2CuO 2Cl 2, we demonstrate that Zhang–Rice singlet excitation, dd excitation, as well as two spin-flip excitations can be distinguished according to their symmetry.

Electronic structure studies ofV6O13by soft x-ray emission spectroscopy: Band-like and excitonic vanadium states

Resonant soft x-ray emission (SXE) spectra of the mixed valence vanadium oxide ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ have been recorded for a series of excitation energies across the V $L$-absorption band. Resonant excitation allows one to distinguish between charge neutral low-energy excitations and continuum-excited, more band-like V $3d$ valence band states in the spectra. We find that the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ consist of two distinct components that can be assigned to nearly pure V $3d$ states, and to V $3d$ states that are strongly hybridized with O $2p$ states, respectively. Band structure calculations of the density functional theory support the assignment of these features. At threshold excitation the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ show strong signatures from excitonic states, the energy dependence of which shows Raman-like behavior. We compare these spectral features in the resonant SXE spectra with cluster model calculations and assign them to $\mathrm{dd}$ excitations and charge-transfer excitations, respectively. Finally, we discuss changes in the V $L$-absorption and emission spectra that take place when changing the sample temperature from 295 K to 120 K. We relate the changes to redistributions in the V $3d$ partial density of states, which occur at the transition temperature for the metal-semiconductor-transition ${T}_{\mathrm{MST}}=150\mathrm{K}$ and find support in our temperature dependent band structure calculations.

Resonant soft X-ray emission spectroscopy of doped and undoped vanadium oxides

Resonant soft X-ray emission (RSXE) spectra of NaV 2O 5, Mo x V 1− x O 2 and V 2O 3 have been recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. Resonant excitation allows us, firstly, to separate V 3d and O 2p projected density-of-states of the valence band and, secondly, to study charge-neutral low-energy excitations due to resonant inelastic X-ray scattering (RIXS). We found that both the V L- and the O K-emission spectra clearly show components originating from O 2p- and V 3d-states, reflecting the high degree of hybridization of the valence band in all compounds. At threshold excitation we observed that NaV 2O 5 spectra are dominated by RIXS whereas Mo x V 1− x O 2 and V 2O 3 spectra show bandlike features, which may be due to differences in the correlation effects of the compounds. We compared the RSXE spectra with cluster model calculations, which gives a good account for NaV 2O 5 whereas the RSXE spectra of the other compounds show RIXS only at certain energies well above the threshold. In fact, we interpret the trend in the RSXE spectra of the Mo x V 1− x O 2 compound system as a successive filling of the (rigid) V 3d band with increasing Mo content.

RESONANT SOFT X-RAY EMISSION SPECTROSCOPY OFV2O3,VO2andNaV2O5

Resonant soft X-ray emission (RSXE) spectra of V2O3, VO2and NaV2O5were recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. The V L- and O K-emission in these vanadium oxide bands possess considerable overlap. By resonant excitation we can tune the energy to the absorption thresholds, thereby eliminating this overlap. Hereby we obtain the V 3d and O 2p projected density-of-states of the valence band. Resonant inelastic X-ray scattering (RIXS) is found to be weak in V2O3, which we explain as being due to its metallic character at room temperature. Vanadium dioxide ( VO2), semiconducting at room temperature, shows considerable RIXS features only at the O K-emission band. Distinct RIXS structures are visible in the RSXE spectra of the insulator NaV2O5. In the emission spectra excited at the V L-thresholds of this ternary vanadium oxide, dd-excitations of the V dxysubband at an energy loss of -1.7 eV are observed. Our observation, that RIXS is stronger for insulators than for metals, should be taken advantage of for studying insulator-to-metal transitions in vanadium compounds in the future.

Valence excitations observed in resonant soft X-ray emission spectra of K2Ni(CN)4·H2O at the Ni 2p edge

Resonant soft X-ray emission spectra have been measured at the Ni 2p edge of a planar nickel complex K 2 Ni(CN) 4 ·H 2 O and interpreted by performing ab initio configuration interaction calculations. This cyano complex shows characteristic metal-to-ligand charge transfer (MLCT) bands in the Ni 2p photoabsorption spectra. Resonant soft X-ray emission via the MLCT intermediate states can be interpreted as valence MLCT excitations within a one-electron picture. On the other hand, resonant soft X-ray emission via the intra-atomic Ni 2p–3d intermediate state corresponding to the white line excitation shows significant electron correlation effects.

Soft X-ray emission spectroscopy of NiAs-type MnTe at the Mn 2p core threshold

Resonant soft X-ray emission spectra of NiAs-type MnTe have been measured near the Mn 2p threshold using synchrotron radiation. The spectra consist of two structures due to elastic and inelastic scatterings and their relative intensity depends strongly on the excitation photon energy. The inelastic scattering peak around 3–4 eV below the elastic scattering peak can be attributed to d-d * excitation. In order to interpret the resonant behaviour, we have calculated the soft X-ray emission spectra of NiAs-type MnTe on the basis of a cluster model including electron-electron interactions and p-d hybridization. The theoretical results reproduce the basic structures of the experimental spectra, through we notice a difference in their relative intensities.