Itsuro Yamamoto

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...

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-...

The Optical Properties of Liquid Chalcogens at High Temperature and Pressure

The temperature and pressure variations of optical reflectivity of liquid S, Se, Te, Se 80 Te 20 , Se 50 Te 50 , and Se 20 Te 80 have been measured in the energy range between 0.5 and 5.4 eV. Optical conductivity σ(ω) has been derived by using an oscillator fitting analysis. The σ(ω) curve for liquid S and Se has a broad peak, which is assigned to the optical transition from the lone pair to the antibonding σ * states on the adjacent site in the same chain. At high temperature and high pressure, there appears the Drude term in the σ(ω) curve of liquid Se. In this region, liquid Se is transformed from semiconducting to metallic state, which is accompanied by changes in the thermodynamic properties. The observed broad peak around 2 eV for liquid Te may suggest that the covalently bonded twofold structure remains on melting.

Photoelectron spectroscopy of small tellurium clusters

Abstract Isolated tellurium clusters (Te n : n =2–9) were produced in a supersonic molecular beam and their vacuum-UV-photoelectron spectra were recorded at a photon energy of hν =8.3 eV by a photoionization–photoelectron–photoion triple coincidence method. The trimer and tetramer were obtained as stable species in the tellurium cluster beam, unlike sulfur and selenium. The spectra of the odd-membered tellurium clusters have a tendency to be split and broadened, in contrast to those of the even-membered clusters. For the clusters with n ⩾5, the spectra of tellurium clusters are similar to those of selenium counterparts, which may suggest a resemblance of the geometric structures between small tellurium and selenium clusters.

Surface Enhanced Raman Scattering of CO Adsorbed on Silver Film

We have observed the surface enhanced Raman scattering on the CO molecules adsorbed on the silver films which were prepared in the ultra high vacuum at 15 K. The molecules chemically adsorb below 120 K. Three intense Raman bands were found to grow at 155, 180 and 2142 cm -1 . With increasing the thickness of the CO overlayer on the silver surface, the peak of the stretching band at 2142 cm -1 shifts to the low energy side, and the intensity and the band width increase. This result clarifies large discrepancies among reported values of the peak and the band width by other researchers. Rare gas atoms of Kr and Xe overlaid on the CO molecules have been found to induce similar coverage dependence to that observed in the case of the CO overlayer. Therefore, the dipole-dipole interaction is not a main cause of the coverage dependence. Chemical effect is considered to be a possible cause.

Quasielastic neutron scattering of liquid A2X2 (A : chalcogen, X : halogen)

Quasielastic neutron scattering measurements were carried out at room temperature for liquid S 2 Cl 2 , S 2 Br 2 , Se 2 Cl 2 and Se 2 Br 2 , which are composed of the molecules having the X-A-A-X type chain unit, X and A being halogen and chalcogen atoms. The measurements were performed at the scattering wave vectors of 0.98, 1.55, 2.32 and 2.66 A -1 by using AGNES spectrometer installed in JAERI. By analyzing the intermediate scattering function, which was obtained by Fourier transform of the observed dynamic structure factor, a fast and a slow relaxation processes have been found. The fast process, whose relaxation time is less than 1 picosecond, may be assigned to fluctuations in the dihedral angle within the molecule, while the slow process, whose relaxation time depends strongly on the molecular weight, may be assigned to the translational diffusion of the molecule.

The electronic and structural changes in the supercooled liquid and glassy As2Se3

We have measured extended X-ray absorption fine structure (EXAFS) and dc conductivity σ of the liquid and glassy As2Se3. Each |F(r)|, the magnitude of the Fourier transform of EXAFS oscillations around As and Se atoms, has a symmetric peak around 2.5 A. The curve fitting analysis indicates the presence of chemical order and no homopolar As–As and Se–Se bonds. When liquid As2Se3 is cooled to the glassy state through the supercooled state, the inter-layer correlation increases and the two-dimensional (2D) As2Se3 network structure transforms to a three-dimensional (3D) one. This transformation is the reason for the change in the temperature variation of the coordination number N around 300°C whose temperature is higher than Tg(=181°C). The change in the slope of log σ versus 1/T around 300°C is assumed to be associated with the appearance of broadened and localized charged defect states near the band tail.

The semiconductor-metal transition of liquid tellurium-arsenic mixtures

The Hall coefficients, RH, conductivity, σ, and the EXAFS spectra of liquid As–Te mixtures containing 0–40 at.% As have been measured up to 600°C. There are three regimes: I (semiconducting state), II (strong scattering state) and III (metallic state) in the temperature dependence of RH. From EXAFS measurement it is found that the As–Te bonds break with increasing temperature, which coincides with a rapid decrease of RH in regime II, and consequently the network structure of liquid As–Te mixtures is transformed into a chain like structure around 500°C. The structural transformation induces the transition from a semiconductor to metal.

X-ray Absorption Near Edge Structure of Selenium Free Clusters

Selenium clusters have been produced in the free space and their X-ray Absorption Near Edge Structure (XANES) have been measured in situ on the Se K-edge in the Total-Ion-Yield (TIY) mode by utilizing the beam line BL-12C of Photon Factory in KEK. From the observed TIY intensities, the XANES spectrum of Se dimer and that of larger clusters such as a pentamer have been deduced. The first peak in the XANES spectrum of the Se dimer is smaller and broader than that of the larger clusters, suggesting that the first peak of the dimer is a superposition of the 1s-π* and 1s-σ* bound-state resonances.

Raman Scattering Study of the Adsorption State of CO on Silver

Three intense Raman bands were observed at 158, 180 and 2142 cm -1 after the exposure of the silver film to about 0.5 L of carbon monoxide (CO) at 80 K. We found that oxygen coadsorbed with CO induces an upward peak shift of the C-O stretching Raman band to a frequency above that of a gas phase CO. The re-enforcement of the bond between CO and the substrate by the presence of the coadsorbed oxygen was confirmed by the observation of the upward peak shifts of the low frequency bands. Discussion on the change in the adsorption state of CO due to the coadsorption is given in relation to an electronegativity of the coadsorbate. The proposed scheme consistently explains not only the peak shift of the C-O stretching band but also the change in the bond strength between CO and the substrate.