Parity violation in resonant inelastic soft x-ray scattering at entangled core holes.

Resonant inelastic x-ray scattering (RIXS) is a major method for investigation of electronic structure and dynamics, with applications ranging from basic atomic physics to materials science. In RIXS applied to inversion-symmetric systems, it has generally been accepted that strict parity selectivity applies in the sub-kilo-electron volt region. In contrast, we show that the parity selection rule is violated in the RIXS spectra of the free homonuclear diatomic O2 molecule. By analyzing the spectral dependence on scattering angle, we demonstrate that the violation is due to the phase difference in coherent scattering at the two atomic sites, in analogy with Young's double-slit experiment. The result also implies that the interpretation of x-ray absorption spectra for inversion symmetric molecules in this energy range must be revised.

Interpretation of the X-Ray Emission Spectra of Liquid Water through Temperature and Isotope Dependence.

The interpretation of x-ray emission spectroscopy (XES) spectra in terms of their sensitivity to the hydrogen bonding and the consequent microheterogeneity in liquid water has been debated over a decade. To shed a light on this problem, we report the theoretical reproduction of the debated 1b_{1} peaks observed in the XES spectra of liquid water using semiclassical Kramers-Heisenberg formula. The essence of the temperature and isotope dependence of the 1b_{1} double peaks is explained by molecular dynamics simulations including full vibrational (O─H stretching, bending, and) modes, rotational combined with the density functional theory and core-hole induced dynamics. Some inconsistencies exist with the experimental XES profile, which illustrates the need to employ a more precise theoretical calculations for both geometry sampling and electronic structure using a more sophisticated procedure.

Hydration of the Zwitterionic and Protonated Forms of Glycine Betaine Probed by Soft X-ray Emission Spectroscopy Coupled with Chemometrics.

Soft X-ray absorption and emission spectra of glycine betaine (GB) have been measured at the O K-edge in neutral and strongly acidic solutions. The absorption spectra of the neutral solutions have a resonance peak at 532.6 eV, assigned to the transition to the π* orbital, whereas in the acidic solutions, the peak is shifted by -0.3 eV. The emission spectra taken as a function of the GB concentration have been analyzed by means of a modified classical least-squares regression method to obtain the hydration number of the solute. The analysis is successful when the emission spectra have been acquired at the energy of a slightly detuned resonance, giving 28 and 24 as the minimum values for the zwitterionic and protonated GB, respectively. The number of 28 accords with the reported values for the number of water molecules in the first hydration layer of the zwitterion and is greater than that obtained by other experimental techniques. The obtained numbers are used to discuss the hydration structure of GB with the aid of ab initio molecular orbital calculations. The hydration structure of the protonated form of GB is explored for the first time.

Hydrogen bonds of the imidazolium rings of ionic liquids with DMSO studied by NMR, soft X-ray spectroscopy, and SANS.

The hydrogen bonds of the imidazolium-ring H atoms of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides ([Cnmim][TFSA], n = 2 to 12 where n represents the alkyl chain length), with the O atom of dimethyl sulfoxide (DMSO) have been elucidated using 1H, 13C, and 15N NMR spectroscopy and soft X-ray absorption and emission spectroscopy (XAS and XES). Density functional theory (DFT) calculations have been performed on an isolated DMSO molecule and two cluster models of [Cnmim]+-DMSO by hydrogen bonding to interpret the XES spectra for the [Cnmim][TFSA]-DMSO solutions. The 1H and 13C NMR chemical shifts of the imidazolium ring showed that deshielding of the ring H and C atoms is moderate as the DMSO mole fraction xDMSO increases to ∼0.8; however, it becomes more significant with further increase of xDMSO. This finding suggests that the hydrogen bonds of the three ring H atoms with the DMSO O atoms are saturated in solutions with xDMSO increased to ∼0.8. The 1H and 13C chemical shifts of the alkyl chains revealed that the electron densities of the chain H and C atoms gradually decrease with increasing xDMSO, except for the N1-bound carbon atom C7 of the chain. The 15N NMR chemical shifts showed that the imidazolium-ring N1 atom which is bound to the alkyl chain is shielded with increasing xDMSO in the range from 0 to 0.8 and is then deshielded with further increase of xDMSO. In contrast, the imidazolium ring N3 atom is simply deshielded with increasing xDMSO. Thus, the electron densities of the alkyl chain may be condensed at the C7 and N1 atoms of [Cnmim]+ by the hydrogen bonding of the ring H atoms with DMSO. The hydrogen bonding of DMSO with the ring results in low-energy shifts of the XES peaks of the O K-edge of DMSO. Small-angle neutron scattering experiments showed that [Cnmim][TFSA] and DMSO are homogeneously mixed with each other on the mesoscopic scale. This results from the strong hydrogen bonds of DMSO with the imidazolium-ring H atoms.

Development of a spectro-electrochemical cell for soft X-ray photon-in photon-out spectroscopy.

We developed a spectro-electrochemical cell for X-ray absorption and X-ray emission spectroscopy, which are element-specific methods to study local electronic structures in the soft X-ray region. In the usual electrochemical measurement setup, the electrode is placed in solution, and the surface/interface region of the electrode is not normally accessible by soft X-rays that have low penetration depth in liquids. To realize soft X-ray observation of electrochemical reactions, a 15-nm-thick Pt layer was deposited on a 150-nm-thick film window with an adhesive 3-nm-thick Ti layer for use as both the working electrode and the separator window between vacuum and a sample liquid under atmospheric pressure. The designed three-electrode electrochemical cell consists of a Pt film on a SiC window, a platinized Pt wire, and a commercial Ag|AgCl electrode as the working, counter, and reference electrodes, respectively. The functionality of the cell was tested by cyclic voltammetry and X-ray absorption and emission spectroscopy. As a demonstration, the electroplating of Pb on the Pt/SiC membrane window was measured by X-ray absorption and real-time monitoring of fluorescence intensity at the O 1s excitation.

Hydration structure of trimethylamine N-oxide in aqueous solutions revealed by soft X-ray emission spectroscopy and chemometric analysis.

The hydration structure of trimethylamine N-oxide (TMAO) in aqueous solutions has been investigated by means of soft X-ray spectroscopy and chemometric analysis. Soft X-ray absorption spectra in the O 1s region have a concentration-dependent shoulder at 533 eV, which is assigned to the 6a1 resonance of TMAO. Soft X-ray emission spectra acquired at this resonance comprise both TMAO and water components, with a prominent peak at 525.6 eV which is assigned to the emission caused by the 5e to O 1s transition. An apparent inverse concentration dependence of around 523 eV suggests that the electronic structure of water is modified by the strong interaction with TMAO. Such an effect has been included in the quantitative spectral analysis, called the classical least squares regression method, to obtain information on the hydration structure of the system. The analysis indicates that nine or more water molecules interact with a TMAO molecule. The present method offers a useful technique for probing the solvation structure around the solute which interacts strongly with the solvent.

Correlation between Soft X-ray Absorption and Emission Spectra of the Nitrogen Atoms within Imidazolium-Based Ionic Liquids.

Soft X-ray absorption spectroscopy (XAS) has been performed on the N K-edge of two imidazolium-based ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][TFSA]) and 1-ethyl-3-methylimidazolium bromide ([C2mim][Br]), to clarify the electronic structures of the ILs. Soft X-ray emission spectroscopy (XES) has also been applied to the ILs by excitation at various X-ray energies according to the XAS spectra. It was possible to fully associate the XAS peaks with the XES peaks. Additionally, both XAS and XES spectra of the ILs were well reproduced by the theoretical spectra for a single-molecule model on [C2mim](+) and [TFSA](-) using density functional theory. The assignments for the XAS and XES peaks of the ILs were accomplished from both experimental and theoretical approaches. The theoretical XAS and XES spectra of [C2mim](+) and [TFSA](-) did not significantly depend on the conformations of the ions. The reproducibility of the theoretical spectra for the single-molecule model suggested that the interactions between the cations and anions are very weak in the ILs, thus scarcely influencing the electronic structures of the nitrogen atoms.

X-ray emission spectroscopy of bulk liquid water in "no-man's land".

The structure of bulk liquid water was recently probed by x-ray scattering below the temperature limit of homogeneous nucleation (TH) of ∼232 K [J. A. Sellberg et al., Nature 510, 381-384 (2014)]. Here, we utilize a similar approach to study the structure of bulk liquid water below TH using oxygen K-edge x-ray emission spectroscopy (XES). Based on previous XES experiments [T. Tokushima et al., Chem. Phys. Lett. 460, 387-400 (2008)] at higher temperatures, we expected the ratio of the 1b1' and 1b1″ peaks associated with the lone-pair orbital in water to change strongly upon deep supercooling as the coordination of the hydrogen (H-) bonds becomes tetrahedral. In contrast, we observed only minor changes in the lone-pair spectral region, challenging an interpretation in terms of two interconverting species. A number of alternative hypotheses to explain the results are put forward and discussed. Although the spectra can be explained by various contributions from these hypotheses, we here emphasize the interpretation that the line shape of each component changes dramatically when approaching lower temperatures, where, in particular, the peak assigned to the proposed disordered component would become more symmetrical as vibrational interference becomes more important.

Solvation dependence of valence electronic states of water diluted in organic solvents probed by soft X-ray spectroscopy.

Knowledge of hydrogen bonds is important in a wide range of scientific fields. However, information on the electronic state of hydrogen bonded molecules in liquids and solutions is still limited. We applied X-ray emission and absorption spectroscopy to observe the electronic state around oxygen of water and investigate the solvent dependence of occupied and unoccupied valence electronic states of water in acetonitrile, 3-methylpyridine, and ethylenediamine at room temperature under atmospheric pressure. Based on the direct comparison of the spectra, the pronounced variation of the unoccupied 2b2 state of water in acetonitrile and 3-methylpyridine was assigned to the difference of the dipole-dipole interaction and the donor hydrogen bond. For ethylenediamine solution, an enhancement of the peak structure in the post-edge region of X-ray absorption spectra and an evident shoulder structure on the higher energy side of the occupied 1b1 state in X-ray emission spectra were observed. Although ethylenediamine has two amino groups that can form the hydrogen bonds as a proton donor, the obtained results indicate that the amino groups of ethylenediamine also act as a proton acceptor in the hydrogen bond between water molecules. A systematic peak intensity variation of the pre-edge peak corresponding to the lowest unoccupied molecular orbital, 4a1, was also found through the comparison of three organic solvents. Since the peak intensity tends to decrease with increasing number of hydrogen bonds, the results can be interpreted by the covalency of hydrogen bonds.

Selective probing of the OH or OD stretch vibration in liquid water using resonant inelastic soft-x-ray scattering.

High-resolution O 1s resonant inelastic x-ray scattering spectra of liquid H2O, D2O, and HDO, obtained by excitation near the preedge resonance show, in the elastic line region, well-separated multiple vibrational structures corresponding to the internal OH stretch vibration in the ground state of water. The energy of the first-order vibrational excitation is strongly blueshifted with respect to the main band in the infrared or Raman spectra of water, indicating that water molecules with a highly weakened or broken donating hydrogen bond are correlated with the preedge structure in the x-ray absorption spectrum. The vibrational profile of preedge excited HDO water is well fitted with 50%±20% greater OH-stretch contribution compared to OD, which strongly supports a preference for OH being the weakened or broken H-bond in agreement with the well-known picture that D2O makes stronger H-bonds than H2O. Accompanying path-integral molecular dynamics simulations show that this is particularly the case for strongly asymmetrically H-bonded molecules, i.e., those that are selected by preedge excitation.

Polarization dependent resonant x-ray emission spectroscopy of D2O and H2O water: assignment of the local molecular orbital symmetry.

The polarization dependence of the split two peaks in the lone-pair region in the x-ray emission spectra has been determined at several different excitation energies for both D(2)O and H(2)O water. In contrast to predictions based on a narrow range of local water structures where the two peaks would be of different molecular orbital symmetry and arise from, respectively, intact and dissociated molecules, we show that the two peaks in the lone-pair region are both of lone-pair 1b(1) orbital symmetry. The results support the interpretation that the two peaks appear due to fluctuations between two distinct different main structural environments.

Ultrahigh resolution soft x-ray emission spectrometer at BL07LSU in SPring-8.

An extremely high resolution flat field type slit less soft x-ray emission spectrometer has been designed and constructed for the long undulator beamline BL07LSU in SPring-8. By optimizing the ruling parameters of two cylindrical gratings, a high energy resolution ΔE < 100 meV and/or an E∕ΔE ~ 10 000 are expected for the energy range of 350 eV - 750 eV taking into account the broadening by the spatial resolution (25 µm) of a CCD detector. A coma-free operation mode proposed by Strocov et al., is also applied to eliminate both defocus and coma aberrations. The spectrometer demonstrated experimentally that E/ΔE = 10 050 and 8046 for N 1s (402.1 eV) and Mn 2p (641.8 eV) edges, respectively.

Hydrogen bonding of water in 3-methylpyridine studied by O 1s X-ray emission and absorption spectroscopy.

O 1s X-ray emission and X-ray absorption spectroscopy is applied to probe hydrogen bonding of water (D(2)O) in 3-methylpyridine. Owing to element selectivity of X-ray spectroscopies the electronic structure of water in the binary mixture was observed selectively. Based on the observed spectral changes associated with hydrogen bonding in O 1s X-ray emission and X-ray absorption spectra, we have investigated the hydrogen bond of the mixture sample over a wide range of D(2)O concentrations (X(D(2)O) = 0.02-1.0) at room temperature under atmospheric pressure.

Pronounced polarization anisotropy in resonant X-ray emission from acetic acid molecules in solution.

Pronounced polarization dependence of the X-ray emission spectrum, which reflects the symmetry of the initial valence molecular orbital, was observed for acetic acid in acetonitrile solution under ambient conditions.

Co oxidation accompanied by degradation of Pt-Co alloy cathode catalysts in polymer electrolyte fuel cells.

The electronic structure of Co ions in Pt(3)Co alloy cathode catalysts for polymer electrolyte fuel cells (PEFCs) under different operation conditions has been investigated using X-ray absorption spectroscopy (XAS). The line shapes of the Co 2p XAS spectra indicate that the Co atoms in the cathode layer are predominantly metallic in all cells, and the spectrum of the first-operated cell is the most metallic among them. The Co 2p XAS spectra slightly change depending on the operation conditions, and the differences in the spectra can be explained by the presence of cobalt oxides. Decomposition analysis of the Co 2p XAS spectra reveals that the operation of Pt-Co PEFCs yields trivalently oxidised cobalt in the cathode layer.

Selective observation of the two oxygen atoms at different sites in the carboxyl group (-COOH) of liquid acetic acid.

A study of the excitation energy dependence of resonant X-ray emission spectra across the O1s absorption edge in combination with X-ray absorption spectroscopy and density functional theory reveals the site-specific valence electronic structure of liquid acetic acid under ambient conditions.

Identification of valence electronic states of aqueous acetic acid in acid-base equilibrium using site-selective X-ray emission spectroscopy.

Occupied valence electronic structures in the neutral and the anionic forms of aqueous acetic acid and their pH dependence under ambient condition are identified for the first time exploiting the site-selectivity of resonant X-ray emission spectroscopy (XES) in combination with calculation based on density functional theory (DFT).

Coronary arteriovenous fistula presenting as chronic pericardial effusion.

In August 1998, the patient, a 75-year-old woman, was diagnosed with pericardial effusion (PE) during an investigation of cardiomegaly. The PE disappeared after the administration of diuretics, but in February 1999, shortness of breath and general fatigue developed, and PE was again present. Diagnostic pericardiocentesis revealed bloody fluid. Chest computed tomography revealed a markedly expanded and tortuous right coronary artery (RCA). Coronary angiography (CAG) confirmed a RCA-coronary sinus fistula, and there was a significant step-up of O2 saturation at the right atrium. Cardiac tamponade developed soon after CAG, so the patient underwent surgical closure of the CAVF. Although a bleeding point was not identified, the PE was disappeared after operation. Histopathologically, parts of the wall of the fistula were quite thin and erythrocytes and lymphocytes had infiltrated the pericardial space. The clinical course and the findings indicate that the CAVF caused chronic PE.

Reproducibility of Brachial Artery Ultrasound Measurements.

Recent findings suggest that systemic artery endothelial function is associated with the preclinical phase of vascular disease and related to traditional atherosclerosis risk factors. Brachial artery diameter changes in response to hyperemia have been proposed recently as a noninvasive tool to assess endothelial function. To evaluate the reproducibility of brachial artery diameter measurements using ultrasound, we studied 12 healthy subjects (eight men and four women, mean age 37 +/- 9 years). An ATL HDI 3000 machine with a 5- to 10-MHz broadband transducer was used to image the right brachial artery at rest, approximately 4 cm above the elbow. Gray scale ultrasound and color Doppler long-axis images were recorded. Brachial arterial outer diameter (i.e., from anterior adventitia to posterior adventitia) and inner diameter (i.e., from anterior lumen-intima interface to posterior lumen-intima interface) were measured in each subject by two observers. An offline analysis system was used to make measurements at end-diastole from four cardiac cycles. Interobserver and intraobserver measurement variabilities (technical error rates) for brachial artery inner diameter were excellent, ranging from 2.5% to 3.8%. However, interobserver technical error rates for outer diameter measurements were significantly greater than those for inner diameter measurements, ranging from 16.3% to 22.1% (P < 0.001), presumably related to the difficulty in accurately defining the adventitial lines. There were no significant differences in interobserver and intraobserver variability for measurements made using gray scale and color Doppler-aided techniques. We conclude that interobserver and intraobserver reproducibility for brachial artery inner diameter measurements made from ultrasound images is excellent.