eV Absorption Band Research Articles

Correlation between 1.5 eV photoluminescence-band and 3.8 eV absorption band in silica glass

The 3.8 eV absorption band constitutes a characteristic feature of silica glass with excess oxygen. Since 1.5 eV PL band is formed in the presence of a 3.8 eV absorption band, we studied features of the PL near 1.5 eV to further understand the origin of 3.8 eV absorption band. The 1.5 eV PL band was found to have unique characteristics such as: (1) being the highest intensity PL band observed for an oxygen surplus sample with a 3.8 eV absorption band present, (2) having a long lifetime of several ms and (3) having a PL spectrum with hyperfine structure. We also studied the relationship between 1.5 eV PL band and 3.8 eV absorption band. Two types of inter-related behaviors were observed in the intensity of 1.5 eV PL band and the amplitude of 3.8 eV absorption band. Using a thermal heat treatment experiment, we found a linear correlation between 1.5 eV PL band and 3.8 eV absorption band. Also, by comparing the effect of irradiation time on 3.8 eV absorption and 1.5 eV PL bands at 20 K, we found that an increase in the irradiation time caused a decrease in the amplitude of both 3.8 and 1.5 eV bands. Similarities were also observed in the spatial distribution of the two bands. The results obtained suggest that 3.8 eV absorption band plays a role in the formation of 1.5 eV PL band.

Investigation of the composite structure of peak 5 in the thermoluminescent glow curve of LiF:Mg,Ti (TLD-100) using optical bleaching

The composite structure of glow peak 5 in LiF:Mg,Ti (TLD-100), has been investigated using optical bleaching by 310 nm (4 eV) light. The decay of composite peak 5 and the growth of peak 4 are described by two exponential components of the same mean lives for both peaks. Using computerized glow curve deconvolution and contrary to previous investigations, it is determined that peak 4 does not decrease in intensity at long bleaching times and is therefore not associated with the 4 eV absorption band. The fast and slow components of the exponential decay are associated with the bleaching of peaks 5a and 5, respectively. Optical bleaching as a function of the temperature of post-irradiation annealing reveals that the conversion efficiency of peak 5a to peak 4 is very high, at a constant value of approximately 30%, whereas the conversion efficiency of peak 5 is no greater than a few per cent. The behaviour of the conversion efficiency as a function of the temperature of the post-irradiation anneal is explained in the framework of the spatially correlated trapping centre/luminescent (TC/LC) centre model. In this model, the TL of peak 5a results from the geminate recombination of a locally trapped electron-hole pair: peak 5 results from recombination via electron diffusion in the conduction band following thermal release of a singly-trapped electron in the TC/LC structure. Peak 4 arises from a singly-trapped hole in the TC/LC structure. The high conversion efficiency of peak 5a to peak 4 arises from direct optical ionization of the electron in the electron-hole pair, leaving behind a singly-trapped hole (glow peak 4), a direct mechanism not subject to competitive processes. Optical ionization of the `singly-trapped' electron (peak 5), however, can lead to peak 4 only via a multi-stage mechanism involving charge carrier transport in the conduction band, a mechanism subject to competitive processes, which leads to an order-of-magnitude decrease in the observed efficiency

Ultrafast photoexcitations in para-hexaphenyl

The photoexcitations in the visible and near infrared spectral region of textured polycrystalline para-hexaphenyl (PHP) films are investigated by ultrafast pump-probe spectroscopy. We observe a strong stimulated emission band at 2.7 eV and photoinduced absorption bands. We assign this features to singlet exciton, triplet exciton, and polaron absorption, using field assisted pump-probe measurements.

Correlation between oxygen-deficient center concentration and KrF excimer laser induced defects in thermally annealed Ge-doped optical fiber preforms

The amplitude of the 5 eV optical absorption band of oxygen-deficient germanium-doped silica samples (3GeO 2:97SiO 2) was varied by annealing in oxygen at 1000°C for different periods of time. Each sample was irradiated with the same dose of 5 eV light from a KrF excimer laser to study the effect of the absorption band amplitude on the photo-induced changes. ESR (Electron spin resonance) and optical absorption measurements were carried out after each annealing and each laser irradiation cycle. We find that samples with an initially higher 5 eV absorption band amplitude exhibit a larger increase in the absorption bands related to the photo-induced paramagnetic Ge E′ and GEC (Germanium Electron Center) when irradiated with the same laser dose. Deconvolution of the measured spectra shows that the concentrations of the photo-induced paramagnetic Ge E′ and GEC defects are well correlated with the concentration of GLPC (germanium lone pair center) defects associated with a 5.14 eV absorption band.

Ultraviolet-photosensitive effect of sol–gel-derived GeO2–SiO2 glasses

Germanosilicate glasses are prepared by a sol–gel method and the ultraviolet (UV) photosensitivity of the glasses is investigated by optical absorption, electron spin resonance, and photoluminescence measurements. A KrF pulsed laser and a Hg lamp are used as UV light sources. The concentrations of intrinsic germanium-related oxygen-deficient centers in the glasses are successfully controlled by sintering conditions; in air or under reduced pressure. One-order larger intensity of the 5 eV absorption band is observed in the sol–gel glass sintered under the latter condition, as compared to a fiber preform of almost the same composition, which is formed by the vapor-phase axial deposition method. The effect of irradiation with KrF laser pulses on sol–gel glass is different from that on preform glass. On the other hand, the Hg-lamp illumination has the same effect on both sol–gel glass and preform glass. Based on these results, we discuss the defect states in sol–gel-derived glass and fiber preform glass and also the mechanisms of photochemical reactions under these two UV light sources.

Optical Absorption and Electron Spin Resonance Studies of Nonlinear Optical Crystal Li3VO4 Grown by the Floating-Zone Method

The optically nonlinear Li3VO4 crystal grown by the floating-zone (FZ) method has been characterized by optical absorption and electron spin resonance (ESR) spectroscopy. The as-grown single crystals grown in gaseous argon exhibit a broad absorption extending from the uv to near-infrared light regions and an absorption band at about 3 eV. Postoxidation reduces the whole optical absorption and reveals an additional 1.9 eV band. ESR detects several signals associated with transition-metal ions in as-grown and reduced crystals, i.e., Fe3+, Mn2+ and Cr3+ ions. The Mn2+ and Cr3+ signals are absent in well-oxidized crystals, while the Fe3+ signal shows no significant change. The reduced crystals have three absorption bands associated with transition-metal impurities and oxygen vacancies. The diffuse reflectance spectra of transition-metal-doped Li3VO4 polycrystals were also measured to examine the effect of the presence of Fe3+, Mn2+ and Cr3+. The origin of the optical absorption in as-grown, oxidized and reduced Li3VO4 are discussed in terms of unexpectedly doped iron-group transition-metals and oxygen vacancies. The 3 eV and 1.9 eV absorption bands and the broad band extending from the uv to near-infrared regions are suppressed in the well-oxidized crystals subjected to repeated zone refining, indicating that multiple zone refining and complete oxidation are important in obtaining optical-quality Li3VO4 single crystals.

Correlation between optical absorption and EPR in high-pressure diamond grown from a nickel solvent catalyst

Abstract There is a general tendency for the magnitude of the W8 electron paramagnetic resonance, attributed to substitutional negatively charged nickel, Nis−, to increase in sympathy with the strengths of the 1.883 eV and 2.51 eV absorption bands in high-pressure synthetic diamond. The ratio of the intensities of the 1.883–2.51 eV bands is not constant, and is generally lower in diamonds with a low Nis− concentration; a better correlation is obtained between the Nis− concentration and the 2.51 eV absorption than with the 1.883 eV absorption. Concentrations of Nis− up to 70 ppm are detected in some diamonds, and the relatively weak absorption in the 2.51 eV system suggests that either the oscillator strength is very low or that the optical transition occurs at a minor defect with a concentration which increases approximately in direct proportion to the concentration of W8 centres. The 1332 cm−1 absorption induced by nickel, but attributed to N+, does not correlate with Nis− in diamonds with low nitrogen concentrations, in which the concentration of Nis− is also small, and the charge compensation mechanism proposed in earlier investigations needs to be reconsidered for such specimens.

The annealing of interstitial-related optical centres in type II natural and CVD diamond

Abstract Optical absorption bands in diamond, associated with the carbon interstitial, show annealing stages at temperatures below that at which the vacancy becomes mobile. These bands include the 1.86 eV, TR12, R11 and 5RL systems. Although the vacancy is not mobile, the intensity of the GR1 band, which is due to transitions at the neutral vacancy, does decrease by about 15–20% during annealing at these relatively low temperatures (typically 420–540 °C). In this paper we show that the 1.86 eV and R11 systems anneal out completely, concurrently with the “rapid” annealing of the GR1 band, and we attribute the behaviour to the migration of the interstitial to the vacancy, followed by mutual annihilation. The process follows first-order kinetics, with an activation energy of 1.68±0.15 eV. The intensities of the 1.86 eV and R11 absorption bands are closely correlated during this annealing, and it is possible that the R11 absorption is produced by transitions to excited states of the 1.86 eV centre. The 5RL and the TR12 absorption bands increase in intensity, following annealing at 420–540 °C, but their annealing behaviours are not simply related to those of the GR1 and 1.86 eV centres.

Photoabsorption of the neutral oxygen vacancy in silicate and germanosilicate glasses: First-principles calculations

The photoabsorption of the neutral oxygen vacancy defects in silicate and germanosilicate glasses has been studied by first-principles quantum chemical techniques. The lowest singlet-to-singlet excitations in these defects occur around 7.0 eV, and involve promotion of an electron from a bonding orbital between the adjacent Si (or Ge) atoms to a diffuse Rydberg-type orbital. Such excitations are too high in energy to contribute significantly to the 5 eV absorption band in silicate and germanosilicate glasses.

On the origin of the 5.0 and 7.6 eV absorption bands in oxygen deficient α-quartz and amorphous silica. A first principles quantum-chemical study

We have studied the electronic structure and the optical transitions of point defects in α-quartz and amorphous silica by means of cluster models and all electron configuration interaction (CI) calculations. A series of diamagnetic defects has been studied. For each center a full geometry optimization has been performed at the Hartree-Fock level of theory. Electronic transitions to the lowest excited states have been computed by performing extensive CI calculations. We found that the 7.6 eV absorption band experimentally observed in the optical spectrum of oxygen deficient silica is due to a single oxygen vacancy. Electronic transitions from a double oxygen vacancy and a two-coordinated Si are also considered in view of their potential role as the origin of one component of the 5.0 eV absorption band.

Role of impurities in the 5.16 eV optical absorption band of Ge-doped silica

The optical absorption band at 5.16 eV and the related photoluminescence were investigated in Ge-doped silica samples as a function of the Ge content to study the relation between impurities and oxygen deficient defects. The data, collected in a wide doping range, showed that the dependence of the intensity of the 5.16 eV absorption band on the Ge content depends on the thermochemical parameters of the preparation process. Further, two distinct ranges of Ge-doping were individuated showing different dependence of the density of optically active center on the impurity concentration.

Computed Optical Absorption and Photoluminescence Spectra of Neutral Oxygen Vacancies inα-Quartz

We report ab initio configuration interaction calculations of the optical transitions of a Si${\mathrm{O}}_{2}$ neutral oxygen vacancy $({V}_{\mathrm{O}})$ using cluster models. The transition from the ${V}_{\mathrm{O}}$ ground state and the consequent radiative decay explain the 7.6 eV absorption and the 4.4 eV photoluminescence bands reported in the literature. We suggest that the transition from a metastable structure of ${V}_{\mathrm{O}}$ is responsible for the 5.0 eV absorption band. The model accounts for the fact that the 4.4 eV fluorescence is stimulated by excitations in both UV and vacuum UV regions with different decay kinetics.

Identification of Multiple Charge Carrier Trapping Centers in Irradiated LiF:Mg,Ti via Optical Absorption Studies: The 2.78 eV and 5.0 eV (F Center) Absorption Bands

Identification of Multiple Charge Carrier Trapping Centers in Irradiated LiF:Mg,Ti via Optical Absorption Studies: The 2.78 eV and 5.0 eV (F Center) Absorption Bands

Correlation between the radiation-induced intrinsic 4.8 eV optical absorption and 1.9 eV photoluminescence bands in glassy SiO2

The controversial relationship between the intrinsic 1.9 eV photoluminescence (the R band), the 2.0 and 4.8 eV excitation/optical absorption bands is examined for 11 specimens with different OH, Cl, and O2 concentrations and with different irradiation histories. The direct relation is found between the intensities of the R-band luminescence measured with the 4.0 eV (IUV) or resonance (1.94 eV) excitation (Ires), although it is slightly superlinear, Ires∝(IUV). The deviation from the exactly constant ratio between the band intensities in different glass samples is attributed mainly to the inhomogeneous broadening effects. Also, nonzero polarization degree, P=−1.5±0.5%, in the R-band maximum region is reconfirmed upon linearly polarized 4.8 eV excitation, substantiating a close relation between the 4.8 eV absorption band and the R band. These results suggest strongly that both of the two absorption bands at 2.0 and 4.8 eV are originated from a single center, and quantitatively agree with the recently proposed optical transition scheme in the nonbridging oxygen hole center.

Origin of the 430 K TL Peak in Thermochemically Reduced Alpha-Al2O3

Thermo- and optically stimulated processes are studied in reduced a-Al2O3 excited by X rays or UV light. It is shown that the 430 K TL peak and OSL band at 2.7 eV is caused by release of electrons from traps followed by recombination mainly at F+ centres and resulting in F centre emission. Traps responsible for the composite 430 K TL peak and induced 2.7 eV absorption band are caused by several kinds of defects of an electronic nature. Production of free electrons by selective UV excitation in the region of the absorption of F centres results in effective filling of all kinds of traps responsible for the 430 K TL peak, whereas free electrons and holes produced by X ray excitation competitively recombine via various localised electronic states, which results in lowered efficiency of the accumulation of the light sum and selective filling of different kinds of traps, responsible for the 430 K TL peak.

Site-selective laser-spectroscopy studies of the intrinsic 1.9-eV luminescence center in glassy SiO2.

The intrinsic 1.9-eV photoluminescence band (the R band) in neutron-irradiated synthetic silica glass has been studied using site-selective photoluminescence excitation in the 2.0 eV absorption band and transient spectral hole-burning techniques. The measurements of the low-energy wing of the zero-phonon line intensity distribution function confirm the predicted nearly Gaussian shape with a peak at 1.93 eV and a half-width of 86 meV. The homogeneous shape of the emission contour (``single-site spectrum'') has been evaluated by a selective saturation method, revealing, a phonon sideband with a peak at 60 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a width of approximately 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The total Huang-Rhys factor is estimated as 1.50\ifmmode\pm\else\textpm\fi{}0.5 and the partial Huang-Rhys factor for the interaction with the 890 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ local vibration is 0.08\ifmmode\pm\else\textpm\fi{}0.04. The analysis of spectral parameters indicates that the R band cannot be due to peroxide or ozonide molecular ions and upholds the attribution of the center to the nonbridging oxygen hole center.

Point defects related to 260 K thermostimulated luminescence in α-Al2O3

Thermo- and photostimulated processes are studied in reduced hydrogen containing α-Al 2 O 3 excited by UV light. It is found that UV excitation in F absorption band at 90 K results in a ionization of the F-centers and capture of released electrons at defects thus producing an anisotropy absorption band at 4.2 eV and the dominant thermoluminescence (TL) peak at 260 K. The 260 K TSL peak is accompanied by complete bleaching of the 4.2 eV absorption band and vice versa-by light stimulation in the region of the 4.2 eV band the 260 K TSL peak disappear and released electrons recombine with F + -centers. Both the effect of the preliminary high-temperature thermal treatment of samples on formation of 4.2 eV-centers and the observed dichroism characteristics allows to conclude that corresponding complex defect contains hydrogen and can involve vacancy pair.

Photo- and thermostimulated processes in α-Al 2O 3

We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al 2O 3 crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al 2O 3 crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al 2O 3 crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al 2O 3 in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.

Oxygen-deficient centres in silica glasses synthesised by a laser distillation method

Laser distillation was used to synthesise silica glasses containing oxygen-deficient centres (ODCs). The dependences of the ODC concentration on the compositions of the starting materials and of the atmosphere were studied. Synthesis under neutral and reducing conditions generated silicon ODCs in a concentration of ~1016 cm-3. Similar silicon ODC concentrations were attainable by quenching from above 1300 K. Distillation purification intended to remove the Ge impurity reduced concentration of germanium ODCs by a factor of ~100. The dependences of the absorption and luminescence spectra of the synthesised glasses on the Ge concentration confirmed that the 5.04 eV absorption band was typical of the silicon ODCs and the 5.16 eV band was typical of the germanium centres.

The formation mechanisms of oxygen deficient defects in synthetic silica glasses

The formation mechanisms of oxygen-deficient defects in synthetic silica glass sintered under vacuum were studied. 7.6 and 5.0 eV optical absorption bands in the glasses were measured with synchrotron-orbital-radiation light, and photoluminescence excited by a KrF excimer laser also was measured. It is concluded that these 7.6 eV absorption defects are generated by elimination of chlorine in the vapor-phase axial deposition soot body, and that the 7.6 and 5.0 eV absorption bands exist with a constant ratio.