eV Optical Absorption Band Research Articles

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.

The origin of the intrinsic 1.9 eV luminescence band in glassy SiO 2

The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm −1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm −1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO 2.

Reaction kinetics of oxygen-deficient centers with diffusing oxygen molecules in silica glass

The decay kinetics of the 7.6 eV optical absorption band of hydroxy-free silica glasses during heating in oxygen gas flow was determined. It was found that annihilation of oxygen-deficient centers, ODC(I)s, agrees with the tarnishing reaction model. The absorption cross-section of that center, σ, was determined to be 7.8 ± 0.5 × 10 −17 cm 2. This value is close to 7.5 × 10 −17 cm 2 which was previously determined spectroscopically by hydrogen-gas treatment of oxygen-deficient silica glass.

Structural origin of the 5.16 eV optical absorption band in silica and Ge-doped silica

The origin of the 5.16 eV absorption band observed in silica and Ge-doped silica was studied using optical and electron spin resonance (ESR) measurements. The band was observed only in samples containing Ge, suggesting that it is related to the Ge impurity in silica, while a lack of correlation between the ESR intensity of the induced hydrogen-associated doublet and the absorption coefficient of the 5.16 eV band indicates that it is not related to two-coordinated Si or Ge. The observation of the absorption coefficient increased as the square root of the Ge concentration demonstrates that the 5.16 eV band is not related to two-coordinated Ge defects but that it is an oxygen deficiency center of the divacancy type associated with Ge.

Preferred concentration enhancement of photobleachable defects responsible for 5 eV optical absorption band in SiO2:GeO2 glass preform by heating in a H2 atmosphere

Heat treatment of a 5GeO2-95SiO2 glass preform in a H2 atmosphere at 500 °C for 70 h increased intensities of an absorption band centered at 5 eV by a factor of ∼3. Intensities of photobleachable component, which is the precursor of UV-induced Ge E′ centers and assigned to a neutral oxygen monovacancy, of the 5-eV band were enhanced by a factor of ∼8 by heating. This increment is ∼3 times as large as that of UV-unbleachable component, which is assigned to Ge2+ centers coordinated by two oxygens (neutral oxygen divacancy).

The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon

The optical absorption band at 7.6 eV, which appears in oxygen deficient pure silica, does not correlate with any ESR signal in non-irradiated samples. Longlasting illumination at 80 K in the range of its absorption leads to an increase of the absorption band at 5 eV. Subsequent heating to 290 K restores the initial absorption. These data can be explained as photodissociation and thermal recreation of a complex defect containing a twofold-coordinated silicon defect. This complex defect is responsible for the 7.6 eV absorption band.

Annealing of the F+ EPR centers and the 2.3 and 2.9 eV optical absorption bands in zinc sulfide crystals

AbstractThe F+ EPR centers and the 2.3 and 2.9 eV optical absorption bands are introduced in ZnS crystals by electron irradiation, neutron irradiation, and Zn‐treatments, and then isochronal and isothermal annealings of these specimens are carried out. It is found that the predominant isochronal annealing stages in the electron‐irradiated, neutron‐irradiated, and Zn‐treated specimens occur at 150 to 360, 300 to 600, and 600 to 800 °C, respectively. However, the F+ centers and the 2.3 eV band are simultaneously annealed. Therefore it is established that the 2.3 and 2.9 eV optical absorption bands are due to the F+ centers. It is also described that the sulfur vacancies are stable below about 600 °C in the Zn‐treated specimens and the migration energy of the vacancies may be 1.4 eV.

Temperature dependence of the half-width of the 2.3 eV optical absorption band in neutron-irradiated ZnS

The absorption curves of the 2.3 eV band in neutron-irradiated specimens were measured from 77 to 180 K and the shape of the band is analysed. The shape function of the absorption band indicates that the half-width of the shape increases with increasing temperature. The band shape and the temperature dependence of the half-width of the 2.3 eV absorption band can be approximately expressed by the simple configurational coordinate model.

Electric-field-induced dichroism of the V − center in MgO

Electric-field-induced alignment of V − centers in MgO crystals has been monitored by the ensuing dichroism in the 2.3 eV optical absorption band. The effective electric dipole moment is μ = 3.4±0.4 e A ̊ , and the centers respond freely to the applied fields even at liquid-helium temperatures. The optical excited state is found to be incompatible with a localized-atomic-orbital description.

Electron paramagnetic resonance and magnetic circular dichroism double resonance of the V− center in MgO

This work presents the results of an experimental and semi-theoretical investigation of the V − paramagnetic colour center in MgO. The magneto-optical properties of this center were investigated by using a combined electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) double resonance technique, at temperatures down to 1·6°K. This allows one to correlate positively the 2·3 eV optical absorption band and the axially symmetric spin resonance spectrum. The MCD spectrum is asymmetric, and appears to be a combination of absorptive and derivative like components. The spectrum follows a spin-1/2 Brillouin function, in H/T, and is quenched by high microwave power at both the parallel and perpendicular g-values. To account for the asymmetric shape of the MCD, an analysis is made of previous arguments which were based on a model in which the hole is localized on a single O 2− ion, next to the vacancy. It is found that this description is inconsistent with the experimental evidence presented, and consequently a delocalized model is proposed, through the use of molecular orbital (MO) theory. Considering a MO scheme which has an A 1 ground state and a set of E-like excited states, it is possible to explain qualitatively many of the magneto-optical characteristics which were experimentally observed.

Faraday rotation studies of F centres in alkaline earth oxides

It has been shown by Kemp and others, using Faraday rotation, that the 3.7 eV optical absorption band in neutron-irradiated calcium oxide is due to the F centre. We have used Faraday rotation to identify the F bands at 3.0 eV in strontium oxide and 2.0 eV in barium oxide. By making careful measurements on samples of optical density less than 2 and using the method of moments analysis we have derived values of λ', the effective spin-orbit coupling constant, for the first excited states of the F centres of − 24 cm −1 for calcium oxide, −185 cm −1 for strontium oxide and − 265 cm −1 for barium oxide. These values are in reasonable agreement with those calculated from a simple model of the first excited state of the F centre.