EPR and Optical Absorption Studies of Radiation-Induced Radicals in Biphenyl Single Crystals

Biphenyl single crystals irradiated with γ rays at liquid nitrogen temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. EPR spectra at room temperature exhibit a triplet characteristic separated by 39 gauss and each line of the triplet further splits to a triplet. The main triplet has been interpreted as being caused by hydrogen added biphenyl radicals. The splitting of each line of the main triplet is consistent with the calculated splittings of 2-hydro-biphenyl radicals and 4-hydro-biphenyl radicals. The presence of 3-hydro-biphenyl radicals could not be judged from the EPR spectra due to the broadness of the lines. In the optical absorption spectra at low temperature, obtained with the light polarized to the a-, b- and c'-axes of the crystals, nine bands were detected. Transition energies were calculated using SCF-C1 molecular orbital theory for the three kinds of the hydrogen added biphenyl radicals. By comparing the experimental spectra with the theoretical transitions of the three kinds of radicals, the bands at 666 nm and 424 nm have been assigned to the transitions of 2-hydro-biphenyl radicals, the 660 nm band to that of 3-hydro-biphenyl radicals, and the bands at 604 nm and 598 nm to the ones of 4-hydro-biphenyl radicals. The decays of these radicals at room temperature are very similar and upon annealing a new absorption band appears at 462 nm.     

Charge Transfer Transitions from Cyclohexadienyl-Type Radicals to Host Molecules in Anthracene Single Crystals

The optical absorption spectra of 1-, 2- and 9-dibenzo-cyclohexadienyl radical in anthracene crystals have been obtained and analyzed. It is shown that the absorption lines consist of vibrational progression and charge-transfer lines for 2- and 9-DBCR. Each line belonging to the former has the same polarization as the most intense 0–0 vibrational line and the latter show polarization character depending on the direction of charge transfer and the transition energy was found to have a coulombic dependence on the distance of charge transfer. It is pointed out that only charge transfer lines were observed for 1-DBCR. The splitting of lines were observed and ascribed either to the resonance transfer interaction between two neighboring molecules situated at an equal distance from the radical or to the site symmetry. It is suggested that the intensity of the charge-transfer lines are borrowed from the intra-radical transition. The result that the charge transfer lines are more intense than the intra-radical lines for 1-DBCR was discussed.     

EPR and Optical Absorption Studies of γ-irradiated Pyrene Single Crystals

Pyrene single crystals irradiated with γ rays at room temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. EPR spectra exhibit a triplet characteristic and each line of the triplet shows characteristic features with rotations of the crystals in a magnetic field. From analyses of these spectra the presence of two kinds of hydrogen added pyrene radicals, 3-H₂-pyrene and 2-H₂-pyrene radicals, is concluded. In the optical absorption spectrum nine main absorption bands have been detected. These absorption bands are compared with the theoretically calculated transition energies with SCF-CI molecular orbital calculations for 3-H₂-pyrene and 2-H₂-pyrene radicals. Reasonable correspondences are obtained between theoretical values and the experimental spectrum.     

Colour centre studies in MoO3 films

The growth of defects in vacuum-evaporated MoO₃ films has been studied by optical absorption in the present work. The as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N₂ temperature The films were found to be amorphous from X-ray and electron-microscopic studies. From these studies it is observed that in addition to point defects, defect aggregates similar to the colloidal centres in silver and alkali halides, are present in these films.     

Spectral Studies on VO2+ doped MPPH Crystals

Magnesium Potassium Phosphate Hexahydrate (MPPH), is analogous to the bio‐mineral struvite. Vanadyl doped MPPH crystals are grown by slow evaporation technique. EPR and optical absorption studies are carried out at room and liquid nitrogen temperatures. Spin‐Hamiltonian, crystal field and bonding parameters are evaluated.     

坩埚下降法生长CaF2单晶的研究

采用坩埚下降法生长了CaF2单晶体,研究了不同条件生长的单晶缺陷和光谱性能.结果表明:当晶体生长过程中进入水等含氧杂质时,所生长的晶体不仅在1500nm附近产生非常宽的OH-两倍振动吸收带,而且在可见-紫外波段也形成强烈的色心吸收带.同时,杂质离子Ce3+的存在也导致晶体出现306nm的吸收带.     

Influence of Substituents in the Solid State of Radical Cations of Two Phenothiazine Derivatives

The optical, EPR and magnetic susceptibility of the cation radicals of two phenothiazine derivatives have been investivated as a function of R₂ and R₁₀ substituents. We have tried to relate all of the above with the conductivity of these compounds and their slight dependence of the conductivity on the temperature. The diffuse reflectance spectra show a charge transfer between 600 adn 1200 nm. This CT band shows the R₂ and R₁₀ substituents influence. The EPR shows the existence of the radical cation and the dependence of the bandwidth on the R₁₀ subsituents. X static measurements give different quantities of free unparied electrons on each radical in the solid state, also related to the R₁₀ substituents.     

Electronic absorption spectra of lead iodide

Optical absorption studies on gel grown single crystals and vacuum evaporated thin film of lead iodide have been carried out at room temperature and liquid nitrogen temperature. The exciton peak in the case of gel grown crystal was observed at 4940 Å whereas for the film at 4900 Å. The presence of lead colloids in the film was revealed through a broad featureless band around 5500 Å. The absorption tails in the temperature range from 80 to 300 K are found to obey Urbach rule. Examining the various power law dependences of the observed optical density on photon energy, it has been established that the transition is of direct type resulting in a band gap of 2.53 ev.     

The effect of implanting nitrogen on the optical absorption and electron paramagnetic resonance spectra of silica

Silica samples (type III, Corning 7940) were implanted with N using multiple energies to produce a layer ∼600 nm thick in which the concentration of N was constant to within ±5%. The optical absorption spectra of the samples were measured from 1.8 to 6.5 eV. Electron paramagnetic resonance (EPR) measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 K to 100 K for most measurements. The components identified in the EPR spectra, based on comparison with reported parameters, were due to E′ centers and peroxy radicals. By comparing the changes in the optical absorption at 5.85 eV with the changes in the concentrations of the various EPR components and with the reports in the literature, we conclude that there is an additional band at 5.7-5.9 eV other than the E′ center band. We conclude that the bands between 2 and 6.5 eV and the EPR spectral components produced by implantation of N are due to radiation damage processes; neither optical bands nor EPR components related to N are detected.     

Radiation-induced defects in fluoride glasses

Fluorozirconate (ZBLA) and fluorohafnate (HBL) glasses have been exposed to ionizing radiation at room temperature, and the resulting defects have been characterized using optical absorption and electron spin resonance techniques. Prominent absorption bands were found to peak in the ultraviolet at 290 nm for the ZBLA glass and at 240 and 310 nm for the HBL glass. Two major electron spin resonance spectra having g values of 1.883 and 2.043 were induced in the HBL glass. Similar electron spin resonance spectra were found in the ZBLA glass. A third spectrum with a g value of 2.008 was also observed in the ZBLA material. The optical and electron spin resonance spectra in these glasses thermally annealed between room temperature and 300°C. Possible models for the defects are discussed.     

Paramagnetic centres in X-irradiated LiNaSO4 single crystals

The EPR and optical absorption spectra of paramagnetic centers produced by X-irradiation in LiNaSO₄ single crystals were investigated. 7 EPR lines are observed at room temperature and 6 more lines – at LNT. The angular dependences of line positions at LNT are studied and the principal g-factor values are defined. The comparison with published data permitted to assign six the most anisotropic lines to ion-radical SO with different orientations in the lattice; two lines to ion-radical SO; slightly anisotropic doublet line – to O ozonide ion; isotropic line with g = 2. 0045 – to ion radical SO.     

The effect of nitrogen doping on amorphous germanium

The effects of nitrogen doping on the electrical and optical properties of amorphous germanium are investigated. It is found that within the low nitrogen concentrations that cause no appreciable change in the optical energy gap, the room temperature conductivity and the B coefficient in the optical absorption show a maximum at the same nitrogen concentration. This behavior is interpreted by a delocalization of the electronic states in the conduction band due to the nitrogen incorporation.     

Characterisation of the Six-coordinated Octahedral Site in Cd(C4H2O4) · 2H2O Crystals

Optical absorption spectra of Co(II) and Ni(II) doped cadmium maleate dihydrate (CMDH) are recorded at room temperature and liquid nitrogen temperature. The UV–VIS–NIR spectra are characteristic of the transition metal ions in solids. The results and analyses of the spectra indicate near octahedral site symmetry for cobalt ion and trigonally distorted octahedral site symmetry for nickel ion. The following crystal field parameters are derived. The IR spectra are characteristic of the host lattice CMDH.     

EPR Studies of defects in silicon

Electron and neutron irradiated, plastically deformed silicon has been studied by EPR. Irradiation resulted in a strong decrease of the EPR spectra produced by the deformation and in the appearance of a new EPR center at dislocations. Thermal annealing lead to a reestablishment of the formerly present dislocation spectra. As most probable explanation is proposed a change of the dislocation strain field by irradiation induced point defect iclusters. – EPR investigations of iron in boron doped Si demonstrated the pairing of all nterstitial iron in FeB pairs during room temperature storage. Thermal treatment at temperatures higher than 100°C lead to the reappearance of Fe. Therefore the time limiting factor for the annealing of FeB pairs should be the precipitation of interstitial iron.     

Growth and Optical Characterization of CdI2 Single Crystals

Single crystals of CdI₂ were grown by employing the horizontal zone refining method. Emission spectra of these crystals were recorded at room temperature and liquid nitrogen temperature. Various emission parameters such as optical gain coefficient, lifetime, gain, stimulated emission cross section were evaluated for the various emission components. The results show high gain at low temperature whereas very low value at rmm temperature. This has been explained by invoking the existence of self trapped excitons in the crystal.     

Spectral Studies of Divalent Copper in Antlerite Mineral

Electron paramagnetic resonance (EPR) spectra on antlerite mineral are recorded both at RT & LNT. Optical absorption spectra are recorded from 200 – 2500 nm at RT. The chemical analysis of antlerite reveals that the concentration of CuO in the mineral is 67.30 wt%. Optical absorption spectrum of antlerite shows four bands, which are mainly due to Cu(II). EPR and optical results of the antlerite sample confirmed that Cu(II) is in rhombically distorted (D_2h) octahedral symmetry with g₁ = 2.36, g₂ = 2.16 and g₃ = 2.07. The NIR spectra of antlerite are attributed to water fundamentals.     

Optical phonon modes and transmissivity in BaWO4 single crystal

Barium tungstate (BaWO₄) single crystal has been grown using Czochralski technique. It belongs to the scheelite structure, forming the space group I 4₁/a at room temperature and the primitive cell contains two molecular units. The polarized Raman spectra were recorded by a micro‐Raman spectrophotometer system in the backscattering geometry. All the observed Raman modes were assigned. The Raman mode at 924 cm^–1, which belongs to the totally symmetrical A_g optical modes, has the strongest intensity and its linewidth is 4.6 cm^–1. The infrared active lattice vibrations have been studied, eight optical modes were observed and assigned. The ultraviolet absorption edge is at 256 nm and the optical transparency range is up to 2500 nm at room temperature. The energy gap E_g of this crystal was obtained from the optical transmission spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoinduced structural changes in antimony polyphosphate based glasses

A photocontraction effect in amorphous films of the binary glass system 0.20 [Sb(PO₃)₃]_n–0.80 Sb₂O₃ has been observed after UV irradiation using the 350.7 nm Kr⁺ ion laser line with 5.0 W/cm². Good optical quality films up to 4.0 μm were deposited on silica substrates at room temperature in vacuum by electron beam physical vapor deposition (EB-PVD) and characterized using WDX, XRD, optical absorption, infrared reflectance, profilometry and atomic force microscopy (AFM) techniques. Very stable glasses were prepared by the melt quenching technique and used as evaporation source for the production of films. The photoinduced structural change (PSC) was observed as a variation of about 6% in the film thickness and this effect is accompanied by a photobleaching of the irradiated area with a blue shift of the optical absorption edge. Otherwise this photoinduced change in the film thickness is very sensitive to the variations in the shape and intensity of the laser beam; therefore several possibilities in optical recording arise from these results.     

Time evolution of a sol–gel process monitored by Mn2+ EPR spectroscopy

The time evolution of the tetraethoxysilane (TEOS)-based sol–gel process was investigated by EPR spectroscopy of a Mn²⁺ spin probe. Analysis of the EPR spectra recorded at room temperature (RT) and liquid nitrogen temperature (77 K) showed that the values of the g-factor (for the central Mn²⁺ sextet) were found to be, within experimental error, identical during the sol–gel process evolution. However, the peak-to-peak half width, ΔB_pp, of the spectral lines recorded at RT, the splitting of the forbidden hyperfine lines (and thus the zero-field splitting parameter, D) recorded at 77 K, and the averaged hyperfine splitting constant, A_av, at both temperatures, were found to change significantly during the various stages of the sol–gel process. These EPR spectral parameters allow us to monitor changes in the TEOS-based sol–gel reaction mixture during the liquid-to-solid-state transition. It was clearly demonstrated that the sol-to-gel transition as well as the hydration and dehydration process of gels could be successfully studied by Mn²⁺ EPR spectroscopy.     

Optical properties of nanocomposites with iron core-iron oxide shell structure

Iron nanoparticles of diameter ∼5 nm were produced within a gel-derived silica glass by reducing a suitable gel composition. By heating these composites in the temperature range 573-973 K, Fe₃O₄ shells of a few nanometer thickness were grown around the iron nanoparticles. Three peaks were observed in the optical absorption spectra of the nanocomposites when they were dispersed in ethyl alcohol. The first one around 300 nm was caused by plasma resonance absorption of unoxidized iron particles; the second was shown to be due to the core-shell structure with different permittivities of the two regions and the third one was ascribed to a d-d transition. Detailed analyses of the second peak showed that the extracted values of electrical conductivity were below Mott’s minimum metallic conductivity for iron in the case of particles with diameters below ∼2.5 nm.