XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Turbidity determination of the critical exponent eta in the liquid-liquid mixture methanol and cyclohexane

The turbidity of the liquid-liquid mixture methanol-cyclohexane has been measured very near its critical point and used to test competing theoretical predictions and to determine the critical correlation-correction exponent eta. By measuring the ratio of the transmitted to incident light intensities over five decades in reduced temperature, we are able to determine that Ferrell's theoretical prediction for the turbidity explains the data with the correlation length amplitude xi0=0.330+/-0.003 nm and critical exponents eta=0.041+/-0.005 and nu=0.632+/-0.002. These values are consistent with the values measured before for xi0 in this system and with the exponents predicted by theory. The data allow five different theoretical expressions to be tested and to select two as being equivalent when very close to the critical point.     

ANTIVIRAL ACTIVITY OF GILVOCARCIN V PLUS UVA RADIATION

Abstract Gilvocarcin V (GV), a coumarin, is a nucleic acid photosensitizer that is phototoxic to bacteria and mammalian cells at picomolar levels in the presence of near-UV radiation (UVA). We evaluated the effectiveness of GV plus UVA for inactivation of several viruses, including herpes simplex virus, type 1 (HSV) and the bacterial viruses φX174, T7, PRD1 and φ6. Some inactivation of the bacterial viruses was observed with UVA radiation alone (4–50% survival at 26 kJ/m²). Additional photosensitized inactivation was observed only with T7 and φ6 at 2.0 μ M GV. On the other hand, HSV was photoinactivated with concentrations of GV three orders of magnitude lower (1.0 n M ). Similar to the case with UV (254 nm) inactivation, the GV-UVA survival curve for HSV indicated multicomponent inactivation kinetics, which could not be explained by photobleaching of GV. The wide range of photosensitivities of these viruses to GV cannot be adequately explained by models based only on viral nucleic acid content or presence of lipid envelopes.     

Investigation of TiO2SiO2 glasses by X-ray absorption spectroscopy

The coordination and nearest-neighbor bond distances of Ti in a series of TiO₂SiO₂ glasses have been quantitatively determined using a combination of XANES and EXAFS measurements about the Ti K-edge at 4966 eV. The glasses covering the range 0.012 to 14.7 wt% TiO₂ were prepared by flame hydrolysis of predetermined mixtures of SiCl₄ and TiCl₄ vapors. At TiO₂ concentrations below ～0.05 wt%, Ti is found in a rutile-like octahedral coordination. With increased TiO₂ content in the glass, a two-site model applies, in which Ti is found predominately in a fourfold site. About ～9 wt% TiO₂, t6he sixfold/fourfold ratio increases appreciably and eventually at ～15 wt% TiO₂, crystalline TiO₂ segregates out as a second phase. The average TiOSi bond angle in these glasses was estimated to be ～159 ° which is slightly larger than the most probable value of 152 degrees for SiOSi in vitreous SiO₂. Within the accuracy of the edge shift measurements all Ti in the glass is in 4+ valence. Finally, various physical properties such as density, optical transparency and thermal expansion are correlated in light of the new structural data for this interesting binary silicate glass system.     

EXAFS studies of glassy and liquid ZnCl2: A comparison with vitreous GeO2

The Zn-EXAFS (extended X-ray absorption fine structure) above its K-absorption edge has been measured for glassy ZnCl₂ at low temperature, through T_g (375 K), and into the supercooled and normal liquid state (mp = 593 K) at 673 K. By Fourier filtering and fitting the normalized glass spectra using α-ZnCl₂ as a model compound, the Zn²⁺Cl^? distance was determined to be (2.34 ± 0.01) Å and the average coordination number about the Zn²⁺ was found to be 5.1 ± 0.8. The latter value agrees with the value of 4.7 nearest neighbors obtained by the molecular dynamics computer simulation study of Woodcock et al., for liquid ZnCl₂ just above its melting point. The agreement is supportive of the notion that short-range order in the glass is reflective of that of the corresponding liquid from which it was quenched. Spectral measurement as a function of temperature indicates that there is considerable dynamic decoupling of the Zn²⁺ from its nearest Cl^? neighbor even below T_g. This is contrasted with similar data in glassy GeO₂ which show that the motion of Ge is strongly coupled with its four oxygen neighbors all the way to T_g. This difference in dynamic disorder substantiates the notion that glassy ZnCl₂, as well as vitreous BeF₂, provides a further weakened structural analog for glassy GeO₂ and SiO₂.     

Systematic atomic number effects in complexes exhibiting ligand luminescence: Part 2. Salicylidene-o-aminophenolates

The room-temperature fluorescence properties of Group IIIA metal complexes with salicylidene-o-aminophenol (SoAP) in aqueous 20% methanol are reported. The data include absorption, emission and excitation maxima, quantum yields and luminescence lifetimes. A systematic heavy atom effect influences the fluorescence decay to an extent sufficient for simultaneous kinetic analysis of ternary metal ion mixtures, based on decayrate measurements.     

Multipoint measurements in optically dense media by using two-photon excited fluorescence and a fiber-optic star coupler

Optical multiplexing with an N × N fiber optic star coupler (with N=3, 4, or 8) and two-photon excited fluorescence is used to achieve multipoint measurements in highly absorbing environments. Differentiation of fluorescence signals from various sampling points was attained by implementing the time-of-flight characteristics of fiber light-guides. Because the transit time of a light pulse through a fiber optic depends largely on the length of the fiber waveguide, fibers of various lengths permit discrimination of the different sampling points in time. With the help of nanosecond time-resolved detection, it was possible to determine the concentration information at several sensing locations simultaneously. Calibration graphs for 2-(1-napththyl)-5-phenyloxazole were linear for all sizes of star coupler, with submillimolar detection limits. Nonlinear excitation of dopants inside the fiber core resulted in an emission signal that depended quadratically on laser power and, as such, was used as an “internal” sensor to correct for power-squared source fluctuations.