Soft X-ray Emission Spectra Research Articles

Synchrotron x-ray analyses of crystalline and electronic structures of carbon nanowalls

The electronic and crystal structures of carbon nanowalls (CNWs) were analyzed using synchrotron x-rays. Although they have branchless graphene sheets with high crystallinities comparable to those of highly oriented pyrolytic graphite, they also have small amount of fluorine atoms and slightly large interlayer spacing of basal plane. Soft x-ray emission spectra indicate that CNWs have similar but not identical electronic structures to HOPG. Such the chemical and crystallographic structures might be one of factors to induce characteristic electrical features of the CNWs.

Vibrationally resolved resonant x-ray emission spectra of diatomic molecules

We report here the measurements of vibrationally resolved resonant x-ray emission spectra of core-excited N2 and O2 molecules in the 1s → π* resonance region. The measured spectra were compared with the simulated spectra calculated based on the lifetime-vibrational interference theory. We find that the specific excitation to the v' = 0 vibrational level in the intermediate core-hole state of N2 molecule leads to clear vibrational structure (V" = 0 − 3) in both of the final electronic X 1Σ+g (participator band) and a 1Πg (spectator band) states. In contrast to that, the excitation to higher-v' vibrational levels gives rise to the complicated structure in the resultant soft x-ray emission spectra. In the case of O2 molecule, we have observed some structures arising from the vibrational progression, but we find that much higher resolution for the spectrometer is needed to observe distinct vibrational levels.

Polarization dependent soft X-ray emission spectroscopy of cobalt nano-islands on a nitrogen-adsorbed Cu(0 0 1) surface

We have investigated the incident polarization dependence of the Co L 3 soft X-ray emission spectra for the single- and double-atomic height Co islands grown on the nitrogen-adsorbed Cu(0 0 1) surface. For non-resonant incident X-ray in the depolarized configuration, a shoulder structure appeared at the high-energy side of the main peak of the spectra while there is no shoulder in the polarized configuration. The shoulder is attributed to the excitation of correlation satellite originated from the periphery Co atoms of the Co nano-islands. The resonant X-ray emission spectrum in the polarized configuration with the excitation energy of the L 3 absorption peak shows a peak located at the high-energy side of the main peak. This additional peak was not observed in the depolarized configuration.

Angular anisotropy of resonant inelastic soft x-ray scattering from liquid water

Resonantly excited soft x-ray emission spectra of liquid water have been measured at two different angles relative to the polarization direction of the exciting radiation. The results demonstrate t ...

Evaluation of High-Resolution X-ray Absorption and Emission Spectroscopy for the Chemical Speciation of Binary Titanium Compounds

For the chemical speciation of binary compounds of tri- and tetravalent titanium, high-resolution X-ray absorption and emission spectra were recorded in different energy regimes in order to evaluate and to qualify both near-edge X-ray absorption fine structure (NEXAFS or XANES) spectroscopy and wavelength-dispersive X-ray emission spectroscopy (WDXES) as spectroscopic methods for this analytical task. A high resolving power in the excitation channel was ensured by use of monochromatic synchrotron radiation provided by BESSY II, where the soft X-ray emission spectra were recorded as well. In the hard X-ray range, emission measurements were performed at SPring-8. For a comparison of the information gained from the various methods, the titanium compounds were classified according to the bonded titanium's oxidation state. Thus, it was possible to distinguish between inner atomic effects due to different oxidation states and external effects related to the respective ligand and the surrounding structure. It becomes evident, that certain compounds, while hardly distinguishable in their Ti-K XANES spectra, still show significant differences in their emission characteristics. On the other hand, some compounds with little difference in their emission spectra are easily distinguished by their NEXAFS structures. Only the combined use of the complementary methods both in the soft and the hard X-ray range allows for a reliable speciation of tri- and tetravalent titanium compounds.

Soft x-ray emission spectra and ferromagnetism in wide-gap doped semiconductors

A study is made of the resonant and nonresonant L x-ray emission spectra of impurities in the semiconducting compounds ZnS:Mn, ZnO:Mn, ZnO:Co, and Co2O:Mn. An analysis of the Mn L2,3 x-ray emission spectra of Zn1−xMnxS (x=0.1–0.3) reveals that the Mn impurities do not form clusters in the ZnS lattice. Studies of the Mn L2,3 spectra and electronic structure of epitaxial films of Zn0.8Mn0.2O annealed at different temperatures show that the cause of the observed suppression of ferromagnetism at T>600°C is segregation of Mn atoms. In this case the Mn atoms occupy both Zn sites and interstitial positions. For Zn1−xCoxO (x=0.02, 0.06, and 0.10) the absence of free carriers that could mediate an exchange interaction between Co ions is established. Mn L2,3 x-ray emission measurements show that in Mn-doped oxides Cu2O synthesized at 650 and 800°C the Mn atoms are found both in interstitial positions and occupy Cu sites, but the configurations of these defects depend on the synthesis temperature. A decrease of the Curie temperature with increasing synthesis temperature may be explained as a manifestation of antiferromagnetic superexchange between substituent Mn atoms via oxygen.

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.

Chemical Analysis of Impurity Boron Atoms in Diamond Using Soft X-Ray Emission Spectroscopy

To analyze the local structure and/or chemical states of boron atoms in boron-doped diamond, which can be synthesized by the microwave plasma-assisted chemical vapor deposition method (CVD-B-diamond) and the temperature gradient method at high pressure and high temperature (HPT-B-diamond), we measured the soft X-ray emission spectra in the CK and BK regions of B-diamonds using synchrotron radiation at the Advanced Light Source (ALS). X-ray spectral analyses using the fingerprint method and molecular orbital calculations confirm that boron atoms in CVD-B-diamond substitute for carbon atoms in the diamond lattice to form covalent B-C bonds, while boron atoms in HPT-B-diamond react with the impurity nitrogen atoms to form hexagonal boron nitride. This suggests that the high purity diamond without nitrogen impurities is necessary to synthesize p-type B-diamond semiconductors.

Electronic structure of Pd 42.5Ni 7.5Cu 30P 20, an excellent bulk metallic glass former: Comparison to the Pd 40Ni 40P 20 reference glass

In-house photoemission and inverse-photoemission spectra (PES and IPES) were measured on Pd 42.5Ni 7.5Cu 30P 20 and Pd 40Ni 40P 20 bulk metallic glasses in order to clarify the origin of excellent glass-forming ability from the viewpoint of electronic structure. Minima are observed for both the metallic glasses at a slightly higher energy than the Fermi level. Incident photon-energy dependent PES spectra were obtained using synchrotron radiation and the Pd 4d partial density of states (DOS) was estimated from the PES data. Soft X-ray emission spectra were also measured near the Ni and Cu 2p 3/2 absorption edges to evaluate, respectively, the Ni and Cu 3d partial DOS in the valence band. The Pd 4d and the Ni and Cu 3d partials in the conduction band were obtained from X-ray absorption spectra around the Pd 3p 3/2 and Ni and Cu 2p 3/2 absorption edges, respectively. It was found that the Pd 4d partial DOS near the Fermi energy largely decreases and becomes localized by replacing the Ni atoms with the Cu atoms, which may be closely related to the excellent glass-forming ability of the Pd 42.5Ni 7.5Cu 30P 20 bulk metallic glass due to a selective formation of Pd–P covalent bonds.

Soft X-ray emission study of Pd–Ni–Cu–P bulk metallic glass

Soft X-ray emission spectra were measured on Pd 42.5Ni 7.5Cu 30P 20 and Pd 40Ni 40P 20 bulk metallic glasses near the Ni and Cu 2p 3/2 absorption edges to evaluate, respectively, the Ni and Cu 3d partial density of states (DOS) in the valence band. The Pd 4d, and the Ni and Cu 3d partials in the conduction-band were also obtained from X-ray absorption spectra around the Pd 3p 3/2, Ni, and Cu 2p 3/2 absorption edges, respectively. The partial DOS of each d state was estimated from the spectra, and the feature of the electronic structure in these glasses were discussed in detail. In particular, it was found that the Pd 4d partial DOS near the Fermi energy largely decreases and becomes localized by replacing the Ni atoms with the Cu atoms, which may closely related to the excellent glass-forming ability in the Pd 42.5Ni 7.5Cu 30P 20 bulk metallic glass.

Optical and X-ray Emission Spectroscopy of High-Power Laser-Induced Dielectric Breakdown in Molecular Gases and Their Mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.

Momentum selectivity and anisotropy effects in the nitrogenK-edge resonant inelastic x-ray scattering from GaN

High-resolution soft x-ray emission and absorption spectra near the N $K$-edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonantly excited x-ray emission spectra show pronounced dispersion of spectral structures, which is attributed to effects of momentum conservation in the resonant inelastic x-ray scattering (RIXS) process. In development of GaN based optoelectronics, momentum selectivity in RIXS can be utilized to control development of band structure in GaN nanostructures.

Valence band electronic structure of V 2O 5 as determined by resonant soft X-ray emission spectroscopy

Resonant soft X-ray emission (SXE) spectra have been measured for a series of excitation energies at the V Lα and O Kα bands of divanadium pentoxide, V 2O 5. In addition, a near-edge X-ray absorption fine structure (NEXAFS) spectrum at the V 2p and O 1s edges has been also recorded to aid the assignment of the features seen in the SXE data. The V Lα SXE spectrum of V 2O 5 consists of two distinct components that can be assigned to the V 3de g-like states strongly hybridized with the O 2p σ-like states and to the V 3dt 2g-like states weakly hybridized with the O 2p π-like states. The resonant SXE spectra recorded at photon energies close to the position of the V L III edge revealed Raman-like features with an energy loss of 7 eV similar to charge transfer states previously observed by Schmitt et al. [T. Schmitt, L.-C. Duda, M. Matsubara, M. Mattesini, M. Klemm, A. Augustsson, J.-H. Guo, T. Uozumi, S. Horn, R. Ahuja, A. Kotani, J. Nordgren, Phys. Rev. B 69 (2004) 125103] for the V 6O 13 oxide.

Electronic structure of the dilute magnetic semiconductorZn0.90Mn0.10Sobtained by soft x-ray spectroscopy and first-principles calculations

Soft x-ray emission and absorption spectra of the Mn ${L}_{2,3}$-edge were obtained from the diluted magnetic semiconductor ${\mathrm{Zn}}_{0.90}{\mathrm{Mn}}_{0.10}\mathrm{S}$, and interpreted with the aid of first-principles computations of the densities of states. The calculations reveal that the Mn $d$ states near the Fermi energy show interesting spin polarization, and are strongly hybridized with the neighboring S atoms, indicative of the $sp\text{\penalty1000-\hskip0pt}d$ hybridization typical of dilute magnetic semiconductors. Resonant inelastic x-ray measurements reveal a 3-eV inelastic energy loss, which is interpreted as a transition from the Mn valence-band $d$ states to a spin-flip multiplet of the $3{d}^{5}$ ground-state configuration, while a 6-eV loss is attributed to observed charge-transfer effects caused by hybridization between the S $p$ states and the occupied Mn $d$ states located a few eV below the Fermi energy.

Resonant Mn L Emission Spectra of Layered Manganite La1.2Sr1.8Mn2O7

Resonant Mn L α,β soft X-ray emission spectra of a colossal magnetoresistive manganese oxide La 1.2 Sr 1.8 Mn 2 O 7 have been measured at the Mn L 2,3 -edges excited by monochromatized undulator radiation. The excitation energy dependence of the spectra has been analyzed in terms of d – d and charge-transfer excitations. The experimental spectra are reproduced by the multiplet calculation for the Mn 3+ configuration, which is consistent with the dominant occupancy of the 3 d x 2 - y 2 orbital and probably a phase separation.

X-ray spectroscopic study of the electronic structure of the high-dielectric-constant materialCaCu3Ti4O12

The element-specific valence- and conduction-band densities of states for the high-dielectric-material $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ have been measured using soft x-ray emission and absorption spectroscopies. Ti ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, Cu ${L}_{\ensuremath{\alpha},\ensuremath{\beta}}$, and O $K$ soft x-ray emission spectra of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ were measured with monochromatic photon excitation on selected energies above the Ti and Cu ${L}_{2,3}$ and O $K$ absorption edges, respectively. X-ray absorption spectra were recorded at the same edges. The electronic structure was also calculated using density functional theory employing the full-potential linearized augmented plane-wave method. Excellent agreement is seen between the results of these calculations and the measured x-ray emission and absorption spectra. This agreement is particularly good at the O $K$ edge where the resonant behavior of the x-ray emission spectrum can be attributed directly to $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-state emission from valence-band O $2p$ states when in resonance with ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ conduction-band O $2p$ states. Resonant inelastic x-ray scattering is observed at the Ti ${L}_{2,3}$ absorption edge and is compared to previous studies of Ti containing perovskite compounds. The role of Cu $3d$ states in determining the band gap of this material is discussed.

Soft X-ray emission spectra of argon atoms doped in solid matrices

Soft X-ray emission spectra in the Ar L region of various solid substrates doped with Ar atoms were measured to investigate the chemical states of the Ar atoms in the solid matrices. Ar ions were implanted into the solid matrices of Si(1 1 1), SiO 2, highly oriented pyrolytic graphite (HOPG), Ti, Cr, Ni, and Cu with an acceleration voltage of 5 kV at room temperature. Soft X-ray emission spectra in the Ar L region were measured using synchrotron radiation. Low-energy tailing was observed at the L 3–M 1 and L 2–M 1 X-ray emission peaks in Ar-doped transition metals. The density of states (DOS) of the Ar 3s orbitals indicated that the low-energy tailed DOS can be formed by hybridization with 3d orbitals in the overpressurized Ar clusters.

Studying 4d–4f transitions in Er using resonant inelastic scattering

The 4d–4f giant resonance in Er 2O 3 is studied using resonant inelastic X-ray scattering, and the results are interpreted within a purely atomic framework. The soft X-ray emission and absorption spectra are measured using tunable synchrotron radiation. The results show significant valency mixing, evidenced by Er 3+, Er 2+, and Er 4+ components to the soft X-ray absorption and resonant emission data. We find strong inelastic resonant scattering and the energy losses are assigned to 4f inner-shell excitations. The results also show resonant mixing of intermediate states that are separated over an energy range of at least 9.3 eV.

Soft X-ray spectroscopy of diluted magnetic semiconductor Ga 1−xM xN (M = Cr, Mn)

We have performed N 1s–2p soft X-ray emission (SXES) and N 1s absorption (XAS) spectroscopy of recently discovered diluted magnetic semiconductor Ga 1−xM xN (M = Cr, Mn). The SXES spectra of doped GaN shows an additional structure on top of the valence band although the rest of the valence band features do not change compared to non-doped GaN SXES spectrum. The XAS spectra of doped samples also show an additional structure in the band gap of GaN, just below the conduction band minimum. Cr substitution results in larger changes in the unoccupied density of states compared to Mn substitution. These results indicate strong hybridization between N 2p and transition metal 3d electrons in Cr or Mn doped GaN. Comparison of the change in the occupied and unoccupied density of states with band structure calculations for the un-doped and doped compositions indicate that the new states correspond to spin split e g and t 2g states.

The L 2:L 3 intensity ratio in soft X-ray emission spectra of 3d-metals

We present L 2,3 X-ray emission spectra of the 3d-metals Ti through Cu. It is found that the ratio of the integrated non-resonant emission intensities of L 2 to L 3 emission, I(L 2)/ I(L 3), is approximately two times smaller than the ratio of the core hole state degeneracies (L 2/L 3 = 1/2). Our atomic calculations show that the L 2L 3M 4,5 Coster–Kronig transition is responsible for the reduction of the I(L 2)/ I(L 3) intensity ratio and explains its gradual decrease with increasing atomic number among the 3d transition metals. For the resonant case we find that accounting for the core–hole interaction and local field effects via TDDFT satisfactorily explains the changes in the I(L 2)/ I(L 3) ratio.