Nobuhiro Kosugi

Local structures of methanol-water binary solutions studied by soft X-ray absorption spectroscopy.

Liquid methanol shows one- and two-dimensional (1D/2D) hydrogen bond (HB) networks, and liquid water shows three-dimensional (3D) HB networks. We have clearly found three different local structures around the methyl group of methanol-water binary solutions (CH3OH)X(H2O)1-X at different concentrations in C K-edge soft X-ray absorption spectroscopy (XAS). With the help of molecular dynamics simulations, we have discussed the concentration dependence of the hydrophobic interaction at the methyl group in the C K-edge XAS spectra. In the methanol-rich region I (1.0 > X > 0.7), a small amount of water molecules exists separately around dominant 1D/2D HB networks of methanol clusters. In the region II (0.7 > X > 0.3), the hydrophobic interaction of the methyl group is dominant due to the increase of mixed methanol-water 3D network structures. In the water-rich region III (0.3 > X > 0.05), methanol molecules are separately embedded in dominant 3D HB networks of water. On the other hand, the pre-edge feature in the O K-edge XAS shows almost linear concentration dependence. It means the HB interaction between methanol and water is almost the same as that of water-water and of methanol-methanol.

X-ray and Electron Spectroscopy of Water

Here we present an overview of recent developments of X-ray and electron spectroscopy to probe water at different temperatures. Photon-induced ionization followed by detection of electrons from either the O 1s level or the valence band is the basis of photoelectron spectroscopy. Excitation between the O 1s and the unoccupied states or occupied states is utilized in X-ray absorption and X-ray emission spectroscopies. These techniques probe the electronic structure of the liquid phase and show sensitivity to the local hydrogen-bonding structure. Both experimental aspects related to the measurements and theoretical simulations to assist in the interpretation are discussed in detail. Different model systems are presented such as the different bulk phases of ice and various adsorbed monolayer structures on metal surfaces.

Electron dynamics in charge-transfer-to-solvent states of aqueous chloride revealed by Cl- 2p resonant Auger-electron spectroscopy

Charge-transfer-to-solvent (CTTS) excited states of aqueous chloride are studied by a novel experimental approach based on resonant inner-shell photoexcitation, Cl(-)aq 2p --> e(i), i = 1-4, which denotes a series of excitations to lowest and higher CTTS states. These states are clearly identified through the occurrence of characteristic spectator Auger decays to double Cl 3p valence-hole states, where the CTTS states can be more stabilized as compared to single Cl(-)aq 2p core excitations and optical valence excitations. Furthermore, we have found for the first time that the CTTS electron e(i) bound by a single Cl 2p hole not only behaves as a spectator e(i) --> e(i), bound by a double valence-hole state before relaxation of the excited electron (i) itself, but also shows electron dynamics to the relaxed lowest state, e(i) --> e(1*). This interpretation is supported by ab initio calculations. The key to performing photoelectron and Auger-electron spectroscopy studies from aqueous solutions is the use of a liquid microjet in ultrahigh vacuum in conjunction with synchrotron radiation.

Self-Assembled Nanowires with Giant Rashba Split Bands

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions.

Observation of the origin of d0 magnetism in ZnO nanostructures using X-ray-based microscopic and spectroscopic techniques

Efforts have been made to elucidate the origin of d(0) magnetism in ZnO nanocactuses (NCs) and nanowires (NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs contain more defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Zn sites. Specifically, the results of O K-edge scanning transmission X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thick region. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge and Zn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation (LDA) + U verified that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies) induced a significant local spin moment in the nearest-neighboring O atoms to the defect center, which was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO.

Strong double excitation and open-shell features in the near-edge x-ray absorption fine structure spectroscopy of ferrocene and ferrocenium compounds.

We have recorded the iron 2p and carbon 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of decamethyl ferrocene (Fe 3d(6)) and ferrocenium hexafluorophosphate (Fe 3d(5)), two low spin compounds with closed- and open-shell electron configurations, respectively. These species have a similar covalent bonding environment but a different electron configuration, allowing us to examine the role of ligand-metal interactions and electron correlation responsible for their NEXAFS spectra. We have interpreted these spectra with the aid of ab initio configuration interaction (CI) calculations. The CI calculations indicate that double excitation is essential to interpret the Fe 2p NEXAFS spectra of not only the open-shell ferrocenium ion but also the closed-shell ferrocene species, even though the ground states of both species are well described within the Hartree-Fock single-configuration approximation.

Systematic Study of Soft X-ray Spectra of Poly(Dg)·Poly(Dc) and Poly(Da)·Poly(Dt) DNA Duplexes

In the present work, we have undertaken a combined experimental and theoretical study of X-ray spectroscopies for DNA base pairs, more precisely near-edge X-ray absorption, X-ray emission, and resonant inelastic X-ray scattering applied to poly(dG).poly(dC) and poly(dA).poly(dT) DNA duplexes. We have derived several conclusions on the nature of these X-ray spectra: the stacking of pairs has very little influence on the spectra; the spectra of a DNA composed of mixed Watson-Crick base pairs are well reproduced by linear combinations of GC and AT base pairs involved; the amine and imine nitrogens show noticeable differences as building blocks in the absorption, emission, and resonant emission spectra. The calculated spectra are in good agreement with experimental results. The ramifications of these conclusions for the use of X-ray spectroscopy for DNA are discussed.

The Chemical Bond in Carbonyl and Sulfinyl Groups Studied by Soft X-ray Spectroscopy and ab Initio Calculations

The polar character of the sulfinyl bond, which determines many of the properties of dimethyl sulfoxide (DMSO), is a result of charge transfer in low-lying π-type orbitals. This characteristic—toge ...

Probing orbital symmetry in solution: polarization-dependent resonant inelastic soft x-ray scattering on liquid micro-jet

Polarization-dependent resonant inelastic x-ray scattering is demonstrated here for liquid acetonitrile, acetone and dimethyl sulfoxide, using the liquid micro-jet technique. Selective excitation to an unoccupied orbital with a specific symmetry at the K-edge x-ray absorption of liquid samples determines the polarization-dependent emission of the occupied states. Considering the well-defined unoccupied molecular orbital configuration and utilizing the results of ab initio molecular orbital calculations, the polarization-dependent anisotropy in resonant inelastic soft x-ray scattering is discussed in a membrane-free configuration.

Construction of the Scanning Transmission X-ray Microscope Beamline at UVSOR

Construction of a scanning transmission x-ray microscope (STXM) beamline is in progress at UVSOR (Okazaki, Japan). To obtain high brilliance with high resolving power for the incident x-rays, the combination of an in-vacuum undulator and a Monk-Gillieson mounting monochromator with a varied line spacing plane grating was adopted. Resolving power, spectrum and size of the incident x-rays of the beamline were discussed through simulations Then the photon flux at a sample of ~108 photons/s at the resolving power of 5,000 in the photon energy range from 100 to 700 eV is expected. As the first results, spectrum of calcium in chalk is shown.