[en] Three simple hydrocarbons: methane, ethane, and ethylene in solid state at the temperature of liquid nitrogen were studied by coincidence Doppler-broadening (CDB) spectroscopy and positron annihilation lifetime spectroscopy (PALS). Difference in the momentum distributions of σ- and π-bond electrons was observed by CDB spectroscopy. Breaking of the weak π-bonds in ethylene due to the positron irradiation and trapping of positrons by anion molecules and/or anion radicals cause inhibition in the Ps formation in ethylene observed by PALS. The breaking of the π-bonds in ethylene was detected also as a significant change in the shape of ethylene to methane CDB ratio with the elapsed experiment time

[en] The structure and bond nature for the ground state of the UF₅ monomer have been investigated by means of relativistic and nonrelativistic discrete-variational (DV) Xα molecular orbital (MO) methods. It is found that the nonrelativistic calculation provides only less than half of the U-F bond overlap population obtained by the relativistic calculation for the monomer with the C_4ν geometry which was determined experimentally. Comparison of the U-F bond overlap population and valence level structures between the C_4ν and D_3h geometries indicates that the stability of the D_3h monomer is almost equal to that of the C_4ν one

[en] The saturation number of the [2+2] four-membered cross-link between adjacent C₆₀ molecules in a photopolymerized C₆₀ film has been determined using the intensity of the C1s shake-up satellite, which originates from the π-π^* transition of π electrons of C₆₀ , by x-ray photoelectron spectroscopy. It was found that six [2+2] rings were formed on a C₆₀ molecule when polymerization was saturated. A possible configuration of the C₆₀ polymer was postulated on the basis of the saturation number and the structure of a pristine C₆₀ film. copyright 1997 The American Physical Society

[en] Time-resolved temperature-dependent transmission changes were measured for both pristine C₆₀ and electron-beam-irradiated C₆₀ polymer films using an optical pump-probe technique. Only the signals obtained for the C₆₀ polymer show a temperature-dependent slow decay, which appears in the low temperature region below ∼60 K. This slow decay component exhibits a monotonic increase in both relaxation time and amplitude with decreasing temperature, providing evidence of gap formation associated with ordering fluctuations

[en] We have examined the infrared (IR) spectra of electron-beam (EB) irradiated C₆₀ films, using in situ IR spectroscopy in the temperature range of 60-300 K. The irradiation-time evolution of the IR spectra shows that two highly intense new peaks finally appear around 565 and 1340 cm^-1 when the EB-induced C₆₀ polymerization was saturated. To determine the cross-linked structure of the polymer explicitly, we have compared the IR spectra with theoretical spectra obtained from the cross-linked structure of all C₁₂₀ stable isomers derived from the general Stone-Wales (GSW) rearrangement, using first-principles density-functional calculations. Since each C₁₂₀ isomer has the same cross-linked structure as that of its corresponding one-dimensional (1D) C₆₀ polymer, the IR modes obtained from the cross-linked structure of C₁₂₀ are close to those obtained from the corresponding 1D polymer. Comparison between the experimental and theoretical IR spectra suggests that the 1D peanut-shaped C₆₀ polymer has a cross-linked structure roughly similar to that of the P08 peanut-shaped C₁₂₀ isomer.

[en] The operation of vitrification process is disturbed by the deposition of platinum group metals (PGMs): Pd, Rh and Ru to the bottom of glass melter and the formation of yellow phase (low viscosity glass) by molybdenum (Mo). The establishment of separation process of PGMs and Mo from high-level liquid waste (HLLW) is desired for the stable operation of vitrification process. We propose a new separation process of PGMs and Mo from HLLW, which consists of 3 steps, -1) a simultaneous adsorption process of PGMs and Mo by metal ferrocyanides, -2) a thermal decomposition process of ferrocyanide adsorbing them and -3) a separation process of them by solvent extraction technique. The adsorption of PGMs and Mo into metal ferrocyanides was tested by a HNO₃ solution with major 7 elements and a simulated HLLW with 26 elements. It was found that aluminum ferrocyanide is suitable for the simultaneous recovery of PGMs and Mo from HLLW. The separation of PGMs and Mo from an acidic solution obtained by the thermal decomposition and the acid leaching of PGMs and Mo was accomplished by the combination of two extraction processes, that is, the extraction process of Mo with an acidic organophosphorus compound, di(2-ethylhexylphosphoric acid) and a synergistic extraction process of PGMs with thio-diglycolamide and dialkylamine. (authors)

[en] Coincidence Doppler-broadening spectroscopy was applied to C60 and C70 fullerenes and other carbon phases, such as nanotubes and graphites. Remarkable differences were observed in the Doppler-broadening of the positron annihilation γ-rays in these materials, which reflect the different densities distribution of the momentum for electrons annihilated with positrons. This would be caused by the different composition of the π and σ electrons in annihilation with positrons. This result shows the sensitivity of the coincidence Doppler-broadening method to determine the density distribution of electrons sampled by positrons. (orig.)

[en] Research highlights: → The highlights of the present paper demonstrate that the maximum solid solubility (MSS) of transition metal (TM) atoms in solid γ-U phase, which was determined experimentally, can be explained in terms of both the TM d orbital energy (Md) and the TMd-U6d orbital overlap populations (OOP), which were obtained using first-principle calculations based on relativistic density-functional theory. Interestingly, when the correlation map between Md and the OOP is made, γ-U/TM alloys with a lower MSS appear in the lower left of the OOP-Md map, whereas those with a higher MSS appear in the upper right. Thus, the map obtained using the OOP and Md is useful for designing not only γ-U/TM alloys but also other alloy systems. - Abstract: The alloying behavior of transition metals (TMs) in solid γ-phase uranium (γ-U), which is expected to be used as fuel for next-generation nuclear reactors, was investigated using the discrete-variational Dirac-Fock-Slater molecular orbital method. Using a model cluster, U₈/TM, as the minimum unit of γ-U/TM alloys, we evaluated the d-orbital energy of the TM (Md), the bond order between the TM and U atoms, and the orbital overlap population (OOP) between the atomic orbitals of the TM and U atoms. We subsequently examined the effect of these quantities on the maximum solid solubility (MSS) of the γ-U/TM alloys. The interaction between the U-6d and TM-d atomic orbitals was found to play a key role in determining the MSS of the γ-U/TM alloys. The magnitude of the MSS can be explained in terms of the stabilization energy, which is affected by the Md and the OOP, formed by d-d orbital interactions. We also mapped the MSS of γ-U/TM alloys using the Md and the OOP as the alloying parameters. These results will assist the quantum design of nuclear fuel materials.

[en] Electronic structures of actinyl nitrate-triethyl phosphate complexes (AnO₂(NO₃)₂.2TEP; An=U, Np, Pu) have been studied, using the relativistic discrete-variational Dirac-Slater molecular orbital method. On the extraction behavior of hexavalent actinyl nitrates, it is known that the distribution ratio decreases in the order U>Np>Pu when neutral organophosphorous agents are used as extractants. To elucidate this variation, the bonding nature between the actinide atom and the donor oxygen atom is examined. It is found that the extractability of the organophosphorous agents decrease with decreasing bond overlap population between the actinide atom and the donor oxygen atom of the extractant. (orig.)

[en] The time-dependent change in the concentration of UF₆ monomers populated in the ground state was monitored in a supersonic Laval nozzle flow with an infrared diode-laser spectrometer in which the frequency of the laser beam was fixed at the v=1<-0 transition (627.7 cm^-1) of the ν₃ vibrational mode of the ²³⁸UF₆ monomer. The concentration of UF₆ monomers in the ground state increased immediately after a single shot from a Raman laser tuned to a vibrational mode of UF₆ clusters (614.8 cm^-1) was applied to the gas in the nozzle. Subsequently, this concentration leveled off and slowly returned to the previous level. These results indicate that the population of UF₆ monomers in the ground state increased as a result of the predissociation of UF₆ clusters vibrationally excited with Raman laser radiation. It is demonstrated that one can utilize this procedure for vibrational predissociation of UF₆ clusters as a technique to increase the concentration of UF₆ monomers in an irradiation zone for molecular laser isotope separation of uranium. (orig.)