Makoto Yao

Deep Inner-Shell Multiphoton Ionization by Intense X-Ray Free-Electron Laser Pulses

We have investigated multiphoton multiple ionization dynamics of xenon atoms using a new x-ray free-electron laser facility, SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, and identified that Xe(n+) with n up to 26 is produced at a photon energy of 5.5 keV. The observed high charge states (n≥24) are produced via five-photon absorption, evidencing the occurrence of multiphoton absorption involving deep inner shells. A newly developed theoretical model, which shows good agreement with the experiment, elucidates the complex pathways of sequential electronic decay cascades accessible in heavy atoms. The present study of heavy-atom ionization dynamics in high-intensity hard-x-ray pulses makes a step forward towards molecular structure determination with x-ray free-electron lasers.

Photo-Induced Phenomena in Isolated Selenium Chains

Photoacoustic spectroscopy, ESR and EXAFS measurements have been carried out on the Se chains isolated in the channels of mordenite crystal with 6.7 A diameter. The optical gap is estimated to be 2.3 eV at room temperature, which is much larger than that of trigonal Se. At low temperatures, illumination with the light having energy near or above the optical gap induces new absorption bands at 1.85 and 2.25 eV, which are associated with paramagnetic defects. Higher energy light (>2.9 eV) shifts the absorption edge to lower energy. The fluctuations in the covalent bond length increase with illumination, though the helical chain structure is preserved. These changes vanish by annealing at room temperature. The phenomena are reversible in the illumination and annealing cycle. The photo-induced absorption and the edge shift are reduced when the induced bands are excited optically.

Microwave spectroscopy of supercritical water

Microwave spectroscopy that can be applied to study the dielectric relaxation of various fluids under high temperature and pressure has been developed in the frequency range up to 40 GHz. By utilizing this new technique, the dielectric relaxation of water has been measured in the temperature and pressure range up to 750 °C and 120 MPa, which corresponds to a density range between 0.05 and 1 g/cm3. The static dielectric constant e(0) is deduced from the time required for a microwave signal to travel through the sample by means of the time domain analysis, and is in good agreement with the literature. The dielectric relaxation time τ is obtained by fitting the experimentally observed microwave transmission rate to the value calculated using the S-matrices on the assumption that the dielectric constant obeys the Debye relaxation. The results of τ at ambient pressure agree very well with previous data. Below about 350 °C, τ rapidly decreases with increasing temperature nearly independent of pressure, while ab...

Dielectric permittivity of room temperature ionic liquids: a relation to the polar and nonpolar domain structures.

We measured microwave transmission and reflection spectra for typical room temperature ionic liquids, [C(4)min][TFSA], [C(4)min][PF(6)], [C(6)min][PF(6)], and [C(8)min][PF(6)], at frequencies between 40 MHz and 40 GHz in the temperature range up to 100 degrees C. The transmission spectra were analyzed using complex dielectric functions, and the static permittivity epsilon(S) was determined as a function of temperature. Applying the effective medium approximation to epsilon(S), we have estimated that the static permittivity of the polar domain is around 20, and that of the nonpolar domain around 2.5.

EXAFS study on selenium-tellurium mixed chains

Extended X-ray absorption fine structure (EXAFS) has been taken on the Se K-edge for isolated Se-Te mixed chains confined in the mordenite channels, as well as for trigonal and amorphous Se-Te mixtures. The Se-Se and Se-Te bond lengths in the Se-Te chain molecules have been determined by utilizing large difference in the wave vector dependence of backward scattering between Se and Te. The Se-Se and Se-Te bonds in the isolated chains are found to be shorter than those in the trigonal mixtures, which suggests that the intrachain covalent bonds are strengthened by reducing the interchain coupling. The Se-Se bond in the mixed chain molecule elongates with addition of Te, while the Se-Te bond changes little.

Dielectric relaxation of lower alcohols in the whole fluid phase

Dielectric relaxation of ethanol and 1-propanol have been measured in a wide fluid phase including the supercritical condition for the first time. The results of the static permittivity e(0) and the dielectric relaxation time τD are presented in the temperature and pressure range up to 670 K and 30 MPa. Kirkwood’s g-factor deduced from e(0) suggests that the static orientational correlation of dipoles becomes prominent at densities above ∼2dc, dc being the critical density. We divide the fluid phase into four regions and discuss the dielectric relaxation mechanism by extending our model previously applied to water [K Okada et al., J. Chem. Phys. 110, 3026 (1999)]. In the vapor and low-density liquid, the dielectric relaxation is governed by binary collision of molecules. In the high-temperature liquid, the molecules that escape from the hydrogen bond (HB) network also contribute to the relaxation, and the HB breaking is promoted by thermal excitation of the intermolecular stretching modes. In the low-temp...

Viscoelastic properties of room temperature ionic liquids

The sound absorption coefficient alpha and sound velocity nu(S) have been measured for 1-alkyl-3-methylimidazolium hexafluorophosphate [Cn mim]PF(6), with n = 8,6,4 and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4 mim]TFSI, at the frequencies of 11.3, 34.9, and 57.7 MHz in the temperature range from 293 to 393 K. From the obtained alpha and available viscosity data, [Cn mim]PF(6) with large n proves to be no longer Newtonian liquids at room temperatures. Applying a Maxwell viscoelastic model with the elastic modulus G of a spring and the shear coefficient gamma of a dashpot to the experimental frequency dependence of alpha, one finds that G is insensitive to n, while the relaxation time tau(= gamma/G), which is on the order of nanoseconds, does depend on n.

Thermodynamic Properties of Expanded Liquid Mercury in the Metal-Nonmetal Transition Range

Simultaneous measurements of the density and electrical conductivity have been carried out for expanded liquid Hg in the temperature and pressure range up to 1500°C and up to 2200 bar. At densities below 10 g/cm 3 the temperature coefficient of the conductivity at constant volume increases appreciably with decreasing density. Anomalous behaviours are found in the thermodynamic properties such as equation-of-state and isothermal compressibility when the metallic Hg is transformed to a nonmetallic state. Structural changes in the metal-nonmetal transition range are discussed in connection with the observed thermodynamic properties.

Electrical Properties of Liquid Te–Se Mixtures at High Temperatures and High Pressures

The electrical conductivity σ and the thermoelectric power S have been measured for liquid Te–Se mixtures in a wide temperature and pressure range. Substantial changes in σ and S are induced by a slight application of pressure. The region where such changes occur is determined can the concentration-temperature plane. It is suggested that the observed semiconductor to metal transition is originated from the structural change.

Second-order autocorrelation of XUV FEL pulses via time resolved two-photon single ionization of He

Second-order autocorrelation spectra of XUV free-electron laser pulses from the Spring-8 Compact SASE Source (SCSS) have been recorded by time and momentum resolved detection of two-photon single ionization of He at 20.45 eV using a split-mirror delay-stage in combination with high-resolution recoil-ion momentum spectroscopy (COLTRIMS). From the autocorrelation trace we extract a coherence time of 8 ± 2 fs and a mean pulse duration of 28 ± 5 fs, much shorter than estimations based on electron bunch-length measurements. Simulations within the partial coherence model [Opt. Lett. 35, 3441 (2010)] are in agreement with experiment if a pulse-front tilt across the FEL beam diameter is taken into account that leads to a temporal shift of about 6 fs between both pulse replicas.