Emission spectra of KBr:Sn2+

Crystals of KBr:Sn²⁺ irradiated in the A₁, A₂, B or D absorption bands exhibit strong emission in the region of 500 nm. The dependence of this emission on excitation wavelength in the A absorption band shows the 500 nm emission band to be a doublet. This doublet structure is due to electrostatic perturbation from a nearby cation vacancy. It is not possible from emission spectra alone to decide on the actual symmetry of the A_T1 and A_T2 centres responsible for the emission doublet but the various possibilities are discussed. Quenching experiments show that a small emission band at 700 nm is due to Sn²⁺ dimer centres. A series of weak emission bands on the high-energy side of the A_T band are ascribed to emission from the relaxed excited B and D states.     

Absorption phenomena in multicomponent phosphate glasses with Mn

The absorption and ESR spectra of multicomponent alumino-phosphate glasses doped with manganese (0·2–20 mol %) has been measured and compared with the absorption spectra of manganous and manganic ions in model solutions. The unirradiated glasses show in ultraviolet region increasing absorption below 200 nm with the shoulder at 235 nm and the absorption bands due to manganous oxygen complexes in octahedral symmetry. They are similar to the absorption bands of Mn²⁺ in concentrated H₃PO₄ acid. In irradiated glasses the bands at 200, 235, 275 (only if Mn is present) and a broad band at 540 nm appear. After the annealing at 450 C all radiation-induced bands disappear and the bands at 235 nm is more pronounced. The Mn³⁺ in H₃PO₄ solution exhibit the absorption spectrum with a weak band at 530 nm and a very intense band at 270 nm. It is therefore proposed that both the 540 and 275 nm bands in irradiated glasses can be assigned to octahedral oxygen complexes of Mn³⁺, i.e. to hole centres. The band at 200 nm which is practically independent of the modifiers (Be, Mg, Ca, Sr and Ba) is, therefore, associated with electron centres (electrons trapped in non-bridging oxygen vacancies). It is suggested that the band at 235 nm in irradiated and annealed glasses is associated with irreversible structural changes.The authors wish to express their appreciation to H.Dvoáková for preparing the solutions and E.Linhartová for careful measurements.     

Oscillator strength of F2 − colour centres in LiF crystal

The absorption cross-section integral and the oscillator strength of the 960 nm absorption band due to F₂ ^– colour centres in LiF crystal is determined at room temperature from the absorption cross-section spectrum of the F₂ ^– centres obtained from saturable absorption studies. A comparison with previous results is made.     

Luminescence behaviour and Raman characterization of dendritic agate in the Dereyalak village (Eski?ehir), Turkey

Results are presented for the cathodoluminescence (CL), X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and simultaneously two thermal analyses of (DTA/TGA) spectroscopy of dendritic agate which occurs in the Pliocene polymictic conglomerates of the Höyüklü Formation, North West of the Dereyalak village (Eskisehir, Turkey). Micro-Raman measurements were performed on dendritic agate and then strong quartz and moganite peaks were identified at 465 and 501 cm⁻¹, respectively. Thermal analysis shows the loss of water and hydroxyl units occurs in 2 distinct stages; at 796 and 808 °C. Spatially resolved CL results at room temperature were recorded for chosen 3 different areas. Grey area (100% SiO₂) displays the lowest CL emission. Brown area (99.7% SiO₂ and 0.3% Fe₂O₃) contains exsolved non-detected ironed phases such as goethite-lepidochrocite to explain the brown colour and the iron point substitutional defects attributed to the 643 nm CL emission. White outer (98.7% SiO₂ and 1.3% Al₂O₃) would be strongly disordered as observed in the “amorphous” Raman spectrum containing as inferred from the spectrum CL on the outer areas, particularly non-bridging oxygen hole centres (NBOHC) (317 nm) and [AlO₄]°/H⁺ (380 nm) centres produced by large amounts of aluminium in the lattice (1.33% Al₂O₃). When it comes to collect the data in the time resolved CL spectrum, at least three broad emission bands were detected in: a green band of low intensity at about 496 nm, intense orange band at about 600 nm, and a red band at 670 nm. The CL emission at 670 nm shows some relationships between the hydroxyl or alkali content and the abundance of O₂ (super 3-) centres and E′₁ centres. Another conspicuous observed feature in the CL spectra of agates is the existence of an orange emission band centred at around 600 nm. The predominance of the yellow CL emission band and the high concentration of E′₁ centres are typical for agates formed by acidic volcanism processes.     

红宝石吸收宽带的强度

红宝石晶体中Cr³⁺与六个近邻O^2-组成一个有严重畸变的正八面体,因而高阶奇晶场与一阶奇晶场有相同的数量级。由于红宝石中Cr³⁺的格位近似保持C_3v对称,T_2u型的高阶场非常小,可以忽略,但T_1u,A_2u型高阶晶场对红宝石吸收谱强度则原则上不能予以忽视,对于Y带和U^⊥带,高阶场有显著贡献,但对U^‖带,所产生的强度非常小。本文考虑了格点奇对称振动对U,Y带强度的贡献,发现U^‖带主要是奇振动与电子耦合产生的吸收谱。由此出发,讨论了红宝石宽带振子强度、带形等问题,得到了与实验相符合的结果。     

Effect of Br− ions on the F-center formation in KCl crystals under u.v. light irradiation

The relative efficiency of the F-center formation, Y_F in KCl crystals has been studied as a function of photon energy of u.v. light between 5.7 and 10.0 eV. Y_F is maximum at the peak of the absorption band due to the localized excitons at Br^? ions, and increases with Br^? concentration. Results suggest an important role of the localized excitons in the F-center formation.     

V centers in single crystal Al2O3

From optical and thermal bleaching experiments it is concluded that the 400 nm absorption band which appears in Al₂O₃ after γ-irradiation is a composite V band. One of its components is attributed to the V^-_OH center which also is responsible for a localized vibrational band at 3316 cm^-11 analogous to the one observed for the V_OH center in MgO. The irradiation also results in electron trapping at Cr³⁺ impurity ions to produce a band at 227 nm. Annealing at 170°C destroys the V^-_OH center by releasing holes which convert the Cr²⁺ to Cr³⁺ with an attendant thermoluminescence.     

Absorptionsmessungen an OH-dotierten NaF-Kristallen im Vakuumultraviolett

A new absorption band in NaF at 8·3 eV (149 nm) is assigned to the absorption of OH^? ions. A further band at 8·8 eV (142 nm) is developed by X-raying the crystal or illuminating it with OH light. This band is supposed to be the absorption of O₂ molecules on lattice sites. Thef-value of the OH-band is estimated to 0·2.     

Optical polarized properties related to the oxygen vacancy in the CaMoO4 crystal

The electronic structures, dielectric functions and absorption spectra for the CaMoO₄ (CMO) crystal with and without oxygen vacancy V_O²⁺ have been calculated using the CASTEP code with the lattice structure optimized. The calculated results indicate that the optical properties of the CMO crystal show anisotropy and its optical symmetry coincides with the lattice structure geometry of the CMO crystal. The calculated absorption spectra indicate that the perfect CMO crystal does not display absorption band in the visible and near-ultraviolet range. However, in this range, the absorption spectra of the CMO crystal containing V_O²⁺ exhibit one peak at about 1.84 eV (673 nm). It predicates that the 680 nm absorption band is related to the existence of V_O²⁺ in the CMO crystal.     

Farbzentren und Kolloide in abschreckend kondensierten Schichten von NaJ,CaF2 und SrCl2

By a simultaneous evaporation of NaI with excess alkali or alkaline earth metal onto a cooled substrate (T _K =20–150 K) highly disordered thin films are produced, which contain electron excess centers up to a concentration of more than 10²⁰ cm^?3. A complex optical absorption band peaking at 600 and 530 nm is found. The 600 nm band is mainly due to unperturbed F centers, whereas the 530 nm band is ascribed to perturbed F centers, whose symmetry is lowered to C_3v by a nearby Frenkel defect. Another optical transition of this F* center appears at about 600 nm in the dichroic spectrum, which is induced by polarized illumination at 20 K. The transition energies of the F* center are calculated by means of a semi-continuum model. A photochemical transformation of F* and F centers into one another is possible at 20 K. AboveT _K =220 K the excess metal is forming colloidal centers. Two absorption peaks at about 600 and 400 nm, respectively, are ascribed to different colloids, which are either embedded in the crystalline material (600 nm) or only adsorbed to the crystallites (400 nm). By annealing up to 350 K a particle growth and transformation take place. Amorphous quenched films of CaF₂ and SrCl₂ with excess alkaline earth metal reveal mainly colloidal absorption (except a weak F band in CaF₂). The colloids disappear by annealing above 150 K. This is believed to be closely connected with the crystallisation of the films.     

Optical properties of Cu+ centers in NaCl

Different aggregation-precipitation states of Cu⁺ have been characterized by absorption bands peaked at 305, 350 and 372–383 nm.The absorption bands at 372–383 nm, observed exclusively in the most doped crystal, have been associated with the Z₁₂, Z₃ excitons of CuCl microcrystals incorporated into the NaCl matrix Their positions shift to low energies with increasing concentration, as expected for a decrease in the stress over the precipitate.The Z1₁₂, Z₃ exciton bands of CuCl microcrystals precipitated in NaCl can be observed by the optical absorption spectrum without reaching saturation Therefore, this technique could be an alternative method to studies of CuCl thin-film depositions or reflectivity of CuCl single crystals.The red emission band observed at 600 nm is a long-lived emission (τ? 29 ms) at variance with the behavior reported for the Cu⁺ emission It is related to energy transfer processes from Cu⁺ to Mn²⁺.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Red-shift in the photostimulation of the X-ray storage phosphor RbBr:Ga+

Abstract

By means of magnetic circular dichroism of the optical absorption and photostimulated luminescence (PSL) the X-ray induced formation of F_A and F centres in RbBr:(Ga⁺, Li⁺) was investigated. It turnes out that RbBr:Ga⁺ co-doped with 1% Li⁺ in the melt reveals the largest red-shift of the PSL excitation bands to 790 nm, whereby the F_A to F ratio increases up to about 10% with decreasing X-ray dose. This is more than statistically expected (6%). High X-ray doses destroy F_A centres. However, up to 15% of the simultaneously generated F centres could be converted into the F_A species by appropriate bleaching with 633 nm light into the F centre absorption band.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

OH impurities in co-doped LiNbO3:Cr :ZnO congruent crystals

Infrared optical absorption has been used to study OH⁻impurities into congruent co-doped LiNbO₃:Cr³⁺:ZnO crystals doped with different Zn²⁺ concentration. The OH⁻ IR absorption spectra present three bands that can be associated with different OH⁻ complex centres available in the lattice. For crystals with lower Zn²⁺ concentrations (<4.7%) only one IR absorption band centred at 2867 nm (3490 cm⁻¹) is reported which is associated with the OH⁻ unperturbed vibration. For crystals with higher Zn²⁺ concentrations (>4.7%), two new bands associated with OH⁻vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm⁻¹) and 2847 nm (3512 cm⁻¹) and can be associated with OH⁻-Zn²⁺ and Cr³⁺(Li⁺)-OH⁻-Zn²⁺(Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr³⁺ centres in the lattice of the doped LiNbO₃:ZnO crystals.     

Pressure-induced delocalization of negative-U centres in semiconducting glasses

Delocalization of the negative-U centres and/or their excitations, strongly localized at ambient pressure, is predicted to occur in semiconducting glasses at the accessible high critical pressure p_c ≈ 10⁵ bar. The phenomenon is shown to exist because of the increase of the centres overlap with growing pressure, which increases the centres concentration. Then, non-localized charge carriers, related to the negative-U centres, become essential at sufficiently high p > p_c, giving rise to dramatic changes in electron (e.g., transport) processes in semiconducting glasses.     

The colouration of CaF 2 crystals by keV AND GeV ions

CaF 2 crystals have been implanted with a variety of ions of widely different energies and mass. Effects have been monitored using optical absorption in the range 120-750 v nm. This includes the vacuum UV region. For 100 v keV ions (Al, Mg, Kr) we observe extrinsic colloid bands in the case of implanted metal ions at high fluences (10 17 v ions v cm 2 ) but no colour centres (F, F 2 etc). For GeV ions (U, Ni) we observe prominent absorption bands in the visible region at fluences of 10 12 v ions v cm m 2 attributed to extrinsic calcium colloids. New optical features are discussed including an absorption band near 185 v nm in the VUV and bands at 604 v nm and 672 v nm in the visible region.     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.     

退火对He注入及随后208Pb27+辐照的Al2O3单晶PL谱的影响

利用高能离子研究了110 keV 的He⁺注入Al₂O₃单晶及随后230 MeV的²⁰⁸Pb²⁷⁺辐照并在不同温度条件下退火样品的光致发光的特性. 从测试结果可以清楚地看到在375 nm，390 nm，413 nm 和450 nm 出现了强烈的发光峰. 经过600 K退火2 h后测试结果显示，390 nm发光峰增强剧烈，而别的发光峰显示不明显. 在900 K退火条件下，390 nm的发光峰开始减弱相反在510 nm出现了较强的发光峰，到1100 K退火完毕后390 nm的发光峰完全消失，而510 nm的发光峰相对增强. 从辐照样品的FTIR谱中看到，波数在460—510 cm^-1间的吸收是振动模式，经过离子辐照后，吸收带展宽，随着辐照量的增大，Al₂O₃振动吸收峰消失，说明Al₂O₃振动模式被完全破坏. 1000—1300 cm^-1之间为Al-O-Al桥氧的伸缩振动模式，辐照后吸收带向高波数方向移动. 退火后的FTIR谱变化不大.