Yoshinori Ohmasa

Structure and Electronic Properties of Te-Se Mixtures under High Pressure

X-ray diffraction and electrical resistance have been measured for amorphous and crystalline Te 1- x Se x mixtures (0≤ x ≤1) under high pressure up to 30 GPa, and four high pressure phases of Te-Se mixtures have been observed: a monoclinic phase with 6 atoms per unit cell (M-I phase); a monoclinic phase with 4 atoms per unit cell (M-II phase); an orthorhombic phase; a β-Po-type phase. It was found for the first time that the M-II phase containing ∼60 at.% Se can be quenched to the atmospheric pressure. X-ray diffraction, EXAFS (extended X-ray absorption fine structure) and optical reflectivity have been measured for the pressure-quenched Te 0.4 Se 0.6 specimens. Pressure-quenched Te 0.4 Se 0.6 belongs to the space group C m and has a puckered layer structure containing planer zigzag chains with period 3. Moreover, the superlattice structure with a unit cell size of 9 lattice constants along the b-axis was found. The EXAFS results suggest the existence of chemical short range order. We discuss the correlat...

Transverse excitations in liquid Sn

Transverse acoustic (TA) excitation modes were observed in inelastic x-ray scattering (IXS) spectra of liquid Sn. The excitation energies and widths of the TA modes are in good agreement with results of an ab initio molecular dynamics simulation. By comparing current correlation spectra between the experimental and theoretical results quantitatively, we have concluded that the TA modes can be detected experimentally through the quasi-TA branches in the longitudinal current correlation spectra. The lifetime and propagation length of the TA modes were determined to be ~0.7 ps and 0.8-1.0 nm, respectively, corresponding to the size of cages formed instantaneously in liquid Sn.

Surface tension of room temperature ionic liquids measured by dynamic light scattering

Using dynamic light scattering techniques, we obtained the surface tension sigma, surface excess entropy S(sigma), surface excess enthalpy H(sigma), and viscosity eta for the following seven room temperature ionic liquids in a wide temperature range from 30 to around 140 degrees C: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide, N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium hexafluorophosphate, and 1-octyl-3-methyl imidazolium hexafluorophosphate. We have found that sigma increases systematically with decreasing the anion size and the alkyl side chain length. On the other hand, S(sigma) and eta increase with decreasing the anion size but decrease with decreasing the alkyl chain length. H(sigma) seems to decrease with increasing the anion size, but it has no clear dependence on the alkyl chain length. We discuss the bulk and surface properties, referring to the Coulomb interactions and van der Waals interactions.

Photoelectron Photoion Coincidence Measurements of Selenium Cluster Beam. II. Photon Energy Dependence

The photon energy dependence of the photoelectron photoion coincidence (PEPICO) spectra near the Se K-edge have been measured for Se 2 and larger species such as Se 5 at the BL-12C in Photon Factory. The PEPICO spectra reveal that the multiply charged ions, Se z + , are produced as a result of the de-excitation process following the inner-shell excitation. Two prominent structures corresponding to the π * and σ * resonances are clearly observed in the energy dependence of the branching ratio from Se 2 , while only one structure corresponding to the σ * resonance can be seen for the larger species. At the σ * resonance, the production of Se + and Se 2+ is enhanced, in comparison with the direct ionization where the core-hole states are completely relaxed before the dissociation. The energies of the π * and σ * resonance relative to the 1 s ionization potential are estimated at -10.0±0.5 eV and -5±1 eV, respectively. In addition, we propose a new method for the size-selective structural analysis of clusters...

Photoelectron Photoion Coincidence Measurements of Selenium Cluster Beam. I. Evidence for the Coulomb Explosion

The photoelectron photoion coincidence (PEPICO) spectra have been measured for Se 2 and larger cluster, “ Se 5 ”, at photon energies of 12.63 keV and 12.68 keV (i.e., on the both sides of the Se K-...

Wetting phenomena for mercury on sapphire

In the present article we make a brief review of our recent studies on the wetting behaviour of mercury on a sapphire substrate. The prewetting phase diagram, and the temperature and pressure dependence of the prewetting order parameter and its fluctuation are presented. The interplay between the wetting transition and the metal-non-metal transition, and the crossover from wetting to critical adsorption are discussed. The experimental techniques that have been newly developed to study the mercury wetting film under high temperature and high pressure are also explained.

Coulomb explosion of K-shell ionized krypton clusters studied by multiple-ion coincidence momentum imaging.

The Coulomb explosion of K-shell ionized krypton clusters with an average size N of 160 has been studied by electron-multiple-ion-coincidence measurements in which the time-of-flight (TOF) of ions was measured by using a position sensitive detector. The authors have sorted the TOF spectra by the number of coincidence ion signals, Ncoin, and found that singly charged fragment ions such as Kr+, Kr2+, and Kr3+ are dominant for Ncoin>or=2, and that multiply charged ions are detected mainly for Ncoin=1. The Ncoin dependence of the peak widths in the TOF spectra reveals that the average momentum of the Kr+ ions increases with Ncoin, while those of Kr2+ and Kr3+ decrease. These results have been more directly confirmed by the momentum imaging measurements. The authors propose that the heavier ions are produced in the central part of clusters where the Coulomb interactions from the surrounding ions are more effectively canceled out due to the higher symmetry.

Electron–Ion-Coincidence Measurements for K-Shell Excited Free Krypton Clusters

The decay processes of inner-core excited krypton atom and free krypton clusters up to = 1640, being the average cluster size, have been studied by means of electron ion coincidence (EICO) measurements utilizing hard X-ray. Unlike the soft X-ray absorption measurements, highly charged daughter ions are observed. With increasing cluster size, the relative abundance of the multiply charged ions decreases and that of singly charged dimer or trimer ions increases. The results may be interpreted if one takes site-dependent decay processes into account: when the core hole is generated on the cluster surface, the proliferated holes due to the vacancy cascade are strongly localized within the X-ray absorbing atom, while the charges are migrated to surrounding atoms when the X-ray absorption occurs inside the cluster.

Vibrational, single-particle-like, and diffusive dynamics in liquid Se, Te, and Te50Se50

Neutron scattering measurements were carried out for liquid Se, liquid Te, and liquid Te50Se50. The results are discussed in the context of semiconductor-to-metal transition. As to liquid Te50Se50, the vibrational modes show a remarkable change when the semiconductor-to-metal transition occurs by temperature increase. A soft stretching mode was observed for the metallic phase and it shows the structural change; the appearance of long bonds in the metallic phase. The vibrational modes for liquid Se and liquid Te were obtained and agreed well with former works. The crossover from collective to single-particle-like regime was also observed for all liquid samples and the transition ranges (Q=4.5–8 A−1) are presented. The diffusive features in the low-Q region and single-particle-like properties in the high-Q region are discussed and are shown to be consistent with the results for the vibrational modes.

A new method for the size-selective EXAFS of neutral free clusters

Recently we have measured Photoelectron Photoion Coincidence (PEPICO) spectra for selenium dimer Se2 and larger species such as Se5 at the beamline BL-12C in Photon Factory. The PEPICO spectra reveal that the multiply charged ions, Se(z+), are produced as a result of de-excitation processes following the inner-shell excitation and fragmented to atomic ions owing to the Coulomb explosion. The branching ratios from the parent clusters to the daughter ions exhibit clear dependence on the size of the parent clusters. In particular, the branching ratios in the non-resonant absorption region are well reproduced by assuming that the charges are randomly distributed within the clusters before the Coulomb explosion. Based upon these findings, we propose a new method for the size-selective EXAFS of neutral free clusters by utilizing the PEPICO measurements.