Quantum beat between two excitonic levels split by spin--orbit interactions in the oxychalcogenide LaCuOS

Degenerate four-wave mixing signals excited by femtosecond laser pulses were measured in LaCuOCh (Ch = S and Se) at 4 K. The signals for LaCuOS exhibit a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states are split by 9 meV, but no beat structure was observed in LaCuOSe, regardless of the laser's energy. The spin--orbit interaction of the Ch ion accompanied by the hybridization of Cu 3d orbitals causes splitting of the exciton levels. Furthermore, the contribution of Ch p orbitals in the valence band maximum is larger in LaCuOSe owing to increased covalency in the Cu--Ch bond when S is replaced with Se.     

Fabrication of nanoparticulate porous LaOF films through film growth and thermal decomposition of ion-modified lanthanum diacetate hydroxide

This paper first reports fabrication of macro/nanotextured rare-earth oxyfluoride films. Usage of ion-modified lanthanum diacetate hydroxide (LDAH) as self-templates was successful in producing nanoparticulate lanthanum oxyfluoride (LaOF) films. LDAH template films were deposited on glass substrates through a chemical bath deposition in solutions composed of lanthanum acetate sesquihydrate, methanol, trifluoroacetic acid, and aqueous ammonia. The LDAH films had a unique, nestlike morphology owing to a two-dimensional hexagonal crystal growth. Modification of LDAH with trifluoroacetate ions led to formation of LaOF after pyrolyzing the template films at temperatures of 400-600 degrees C in air. The resultant LaOF films had a nanoparticulate porous microstructure, maintaining the morphology of the original LDAH template films. It was also successful to incorporate Eu3+ ions into LaOF through deposition of the LDAH film in a solution containing europium acetate tetrahydrate. The characteristic photoluminescence from Eu(3+) was observed with an ultraviolet-light excitation at 273 nm, indicating that Eu3+ was homogeneously distributed in LaOF host crystals. Thus the ion-modification of LDAH was also demonstrated to be a useful method for preparing nanostructured rare-earth oxyfluoride materials havingvarious cationic compositions.     

Diffusion and reactions of hydrogen in F2-laser-irradiated SiO2 glass

The diffusion and reactions of hydrogenous species generated by single-pulsed F2 laser photolysis of SiO-H bond in SiO2 glass were studied in situ between 10 and 330 K. Experimental evidence indicates that atomic hydrogen (H0) becomes mobile even at temperatures as low as approximately 30 K. A sizable number of H0 dimerize by a diffusion-limited reaction into molecular hydrogen (H2) that may migrate above approximately 200 K. Activation energies for the diffusion, inherently scattered due to the structural disorder in glass, are separated into three bands centered at approximately 0.1 eV for free H0, approximately 0.2 eV presumably for shallow-trapped H0, and approximately 0.4 eV for H2.     

Electronic structure of oxygen dangling bond in glassy SiO2: the role of hyperconjugation

The electronic structure and the nature of optical transitions in oxygen dangling bond in silica glass, the nonbridging oxygen hole center (NBOHC), were calculated. The calculation reproduced well the peak positions and oscillator strengths of the well-known optical absorption bands at 2.0 and 4.8 eV, and of the recently discovered absorption band at 6.8 eV. The 2.0 eV band was attributed to transition from the sigma bond between Si and dangling oxygen to nonbonding pi orbital on the dangling oxygen. The uniquely small electron-phonon coupling associated with the 2.0 eV transition is explained by stabilization of Si-O bond in the excited state by hyperconjugation effects.     

Non-Basic Solution Routes to Prepare ZnO Nanoparticles

Nanocrystalline ZnO particles were prepared from alcoholic solutions of zinc acetate dihydrate without using base such as NaOH or LiOH through a colloid process carried out at a low temperature of 60°C. A comparative study of chemical reactions from zinc acetate dihydrate to ZnO was made using different types of monool solvents, i.e. methanol, ethanol, and 2-methoxyethanol. It was revealed that layered hydroxide zinc acetate was formed as an intermediate and its transformation into ZnO was a key reaction step in any of the solutions. Reaction time necessary for the precipitation of ZnO was greatly influenced by the solvents used. Methanol was useful for the preparation of the ZnO nanoparticles, which were chemically pure in terms of cation impurities and exhibited green photoluminescence by the ultraviolet excitation.     

Elucidation of the stereochemistry of Tl+ in oxide glasses by electron spin resonance spectroscopy

The relation has been elucidated between the stereochemistry of Tl⁺ and the chemical composition of host oxide glasses by measuring the ESR of paramagnetic Tl²⁺ induced with γ-irradiation at 77 K. The spectra have been analyzed by using a new solution of the spin-Hamiltonian developed by the authors. The results indicate: (1) there are two sites for Tl⁺ in a wide variety of oxide glasses (site I and site II). Tl⁺ in site I has a heavily distorted ligand field (lone pair electrons of Tl⁺ occupy a s-p(-d) hybridized non-bonding orbital extruding in one direction). The geometry of the Tl⁺-complex in site II is nearly spherically-symmetric (lone pair electrons locate in an anti-bonding sigma orbital to which the Tl 6s atomic orbital mainly participates); (2) site I appears in the glasses where both the bridging and non-bridging oxygens coexist. Site II is observed in the other glasses; (3) a drastric change in lineshape of Tl²⁺ induced with 77 K irradiation was observed upon thermal annealing. A structural model for this change was proposed; the Tl²⁺ with an excess plus charge is stabilized by the cooperative motion of the alkali ion pairing with Tl²⁺ and intervening non-bridging oxygens, the alkali being repelled and the oxygens being attracted.     

Microporous crystal 12CaO x 7Al(2)O(3) encaging abundant O(-) radicals

Extremely high concentrations (>1020 cm-3) of active oxygenic radicals, O- and O2-, have been created in the zeolitic crystal, 12CaO.7Al2O3 (C12A7), which can accommodate anions in its cavities. An increase in oxygen pressure and a decrease in water vapor pressure at high temperature enhance the formation of the radicals. The oxidation of Pt is observed on the surface of the material as a result of reaction with the active oxygens.     

Thermodynamics and kinetics of hydroxide ion formation in 12 CaO x 7 Al2O3

We have examined the thermodynamics and kinetics of hydroxide (OH-) ions that formed in cages of 12 CaO x 7 Al2O3 (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O2-) ion in the cage and an H2O molecule in the atmosphere to form two OH- ions in the cages. To simply and exactly quantify the OH- content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O2- ion in the cage. Consequently, high solubility of OH- ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH- ions is the rate-determining process.     

Response of polyacrylamide—acyclic poly(oxyethylene)-coated ion-selective electrodes to alkali and alkaline earth metal ions in propylene carbonate and its thermodynamic application

The response of ion-selective electrodes with a membrane of polyacrylamide (PAA) coupled to acyclic poly(oxyethylene) neutral carriers to lithium, sodium, potassium, magnesium and barium ions in propylene carbonate (PC) was investigated. Tetraethylene glycol monododecyl ether (POE4) and hexaethylene glycol monododecyl ether (POE6) were used as the acyclic poly(oxyethylene). Both the PAA-POE4 and the PAA-POE6 electrodes showed a more rapid dynamic response in PC than that in acetonitrile. Nernstian responses to lithium, magnesium and barium ions were obtained with the PAA-POE4 electrode. The selectivity coefficients, logk_Ba²⁺,M^x+, for lithium, sodium, potassium and magnesium ions vs. barium ion obtained in PC with the PAA-POE4 electrode were 3.6, 0.23, 0.02 and 1.1, respectively. The PAA-POE4 electrode was applied to obtain the successive formation constants of the barium ion in PC with N, N-dimethylacetamide (DMA). From the successive formation constants obtained in PC-rich solutions, the Gibbs free energies of transfer of the barium ion from PC to DMA and to PC-DMA mixtures were calculated. The electrode was also used to obtain directly the Gibbs free energies of transfer of the barium ion from PC to PC-DMA mixtures. The calculated values of the free energies were in good agreement with the values obtained experimentally, suggesting that the electrode responded to variations in solvation energy for the barium ion.     

Probing interface properties of nanocomposites by third-order nonlinear optics

We describe the exploitation of third-order nonlinear optical response — particularly nonlinear absorption and the nonlinear index of refraction — to probe interface dynamics, modifications and relaxation processes in granular materials consisting of metal quantum dots embedded in such dielectrics as fused silica and sapphire. Many features of these materials can be interpreted in terms of the quantum-mechanical model of the particle-in-a-box. Electronic and thermal relaxation processes in these novel nanocomposites are dominated by interactions of conduction-band electrons at the boundary between the quantum dot and its surrounding host material. Experimental examples presented include measurements of thermal and electronic relaxation rates, dephasing due to electron collisions at the nanocluster surface, effects of local structural order, changes in the saturation parameter due to chemical modification of the substrate, and one-and two-dimensional heat-transfer effects.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.