Electron transfer from excited F centres to F+, F+A and F+AA centres in CaO studied by optically detected magnetic resonance (ODMR)

ODMR measurements via the magnetic circular polarization of the emission (MCPE) of F_A(Mg) centres in CaO at 1.6 K showed besides the known F_A-ODMR lines a non zero MCPE, which vanishes at g ? 2. From the dynamical behaviour of this MCPE and the F_A luminescence is shown that an electron tunneling takes place from the excited T_1u F centre state to F⁺_A centres forming excited F_A centres. The ground state back tunneling rate is 10⁷ times smaller and cannot explain the observed electron transfer cycle. The back tunneling is also going via excited states. The results can be understood assuming a random centre distribution. A new luminescence band at 785 nm could be identified by ODMR as being due to tetragonal F_AA centres. The electron transfer is also observed between F and F⁺ and F⁺_AA centres.     

TSL, OSL and ESR studies in ZnAl2O4:Tb phosphor

ZnAl₂O₄:Tb phosphor was prepared by combustion synthesis. ZnAl₂O₄:Tb exhibits three thermally stimulated luminescence (TSL) peaks around 150, 275 and 350 °C. ZnAl₂O₄:Tb exhibits optically stimulated luminescence (OSL) when stimulated with 470 nm light.Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in ZnAl₂O₄:Tb. Two defect centres are identified in irradiated ZnAl₂O₄:Tb phosphor and these centres are assigned to V and F⁺ centres. V centre appears to correlate with the 150 °C TSL peak, while F⁺ centre could not be associated with the observed TSL peaks.     

Luminescence of Eu2+-activated Cs(Ca,Mg)F3 and Rb(Ca,Mg)F3

In the system CsCa_0.95?xMg_xEu_0.05F₃ a new phase has been found with x ≈ 0.7, which gives efficient blue luminescence. For RbCa_0.95?xMg_xEu_0.05F₃ efficient green luminescence is obtained at x ≈ 0.5.     

Self-consistent analysis of absorption and emission data forF A centres in alkali halides

Summary A theory which explains self-consistently the emission and the polarization rotation of transmitted light has been developed forF _A centres in crystals with cubic symmetry. Azimuthal dependences ofF _A luminescence in alkali halides are very informative for determining the electron-phonon interaction as well as the parameters related to the structural and optical quality for coulour centre lasers crystals. The theoretical results are applied to the interpretation of the luminescence properties ofF _A (Li) centres in KCl, RbCl and KF.     

Behaviour of F and Fz Centres Under Thermal Stimulation in KCl:Eu2☎ Irradiated with Ionizing and UV Radiation

The optical absorption (OA) of F and F_z, centres in beta-and UV-irradiated KCl: Eu^2? have been studied. The spectra measured at room temperature and during the subsequent heatings up to 600 K, were analyzed by a deconvolution procedure. The OA bands of the F and F_z centres are well described respectively by asymmetric and symmetric gaussiane with parameters that are linearly dependent on the temperature. The thermal stability of the F_z centres correlates with the low-temperature thermostimulated luminescence (TL) peaks. The stability of the F centres is determined at least by two processes. Part of the F centres is destroyed together with the F_z centres. The remaining ones are destroyed during further heating up to 470 K, correlating with the TL peak at 425 K (at heating rate of 0.1 K/s).     

On the nature of optically active centres in selfactivated luminophors of the CaWO4 type

The evidence in favour of the assumption that impurities of V group elements creates defects of the MeO′₄ type (centres of luminescence), Me?₃ and V_?, or WO₃ (capture centres) which are responsible for recombination afterglow different from the steady-state luminescence spectral distribution (Me = As, Sb, Nb, Ta; V₀ - oxygen vacancy) are presented. These same impurities, and phosphorous, lead to quenching of luminescence observed as afterglow with excitation quanta greater than 6.3 eV. This corresponds to the valued of the energy gap. Experimental data together with results of thermodynamic analysis lead to the conclusion that the luminescence of CaWO₄ and of other undoped oxygen containing compounds of transition elements luminophors is caused by direct self-activation connected with ability of these elements to convert spontaneously into a lower valency state and to form variable phases (non-stoichiometric compounds). It is proposed that in case of CaWO₄ centres of luminescence are formed by W⁵⁺.     

EPR spectra of colour centres in x-irradiated BaCl2:K+ and BaCl2:Ag+

X-irradiation at 80°K leads to the formation of V_K centres and, in addition, perturbed F centres in the case of BaCl₂:K⁺. The V_K centres spectra exhibit a superhyperfine structure. A warming to 130°K of crystals X-irradiated at 80°K causes the V_K centres to be perturbed, and besides leads to the formation of (AgCl₄)^2? complexes in BaCl₂:Ag⁺.     

Photoluminescence of F2+2 centres in additively coloured magnesium oxide

The principle luminescence bands excited in additively coloured MgO by radiation in the wavelength range 170–400 nm are observed at wavelengths of 520, 475, 441 and 375 nm. Polarised luminescence and uniaxial stress measurements on the 441 nm band, the radiative lifetime of 25 msec at 1.6 K and temperature dependence of luminescence intensities of the 375, 441 and 375 nm bands are consistent with the 441 nm band being due to ³B_1u → ¹A_g transitions of the F²⁺₂ centre.     

Vacuum ultraviolet luminescence from a micro-pulling-down method grown Nd:(La0.9,Ba0.1)F2.9

Vacuum ultraviolet (VUV) luminescence from a Nd³⁺:(La_1−x,Ba_x)F_3−x (x=0.1) and a Nd³⁺:LaF₃ single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F₂ laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.     

Effect of the α-F̃1 center interaction on the luminescence lifetime of the F center

The presence of α and F′ centers in a F coloured KCl crystal produces an increase of the lifetime of the luminescence excited with F light. One can exclude that the increase of the luminescence lifetime arises from the contribution of an F′ excited state. The increased lifetime is studied at temperatures lower than 80°K and for some α center concentrations as a function of the initial F center concentration. The results are possibly due to the perturbation of the wavefunction of the F excited state by the α centers present after the F → F′ conversion.     

Investigation of defect centres responsible for TL/OSL in MgAl2O4:Tb

Magnesium aluminate doped with Tb³⁺ (MgAl₂O₄:Tb³⁺) was prepared by combustion synthesis. Three thermoluminsence (TL) peaks at 120, 220 and 340 °C were observed. PL and TL emission spectrum shows that Tb³⁺ acts as the luminescent centre. Optically stimulated luminescence (OSL) was observed when stimulated by 470 nm blue light.Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the TL and OSL processes in MgAl₂O₄:Tb³⁺. Two defect centres were identified in irradiated MgAl₂O₄:Tb³⁺ phosphor by ESR measurements which was carried out at room temperature and these were assigned to V and F⁺ centres. V centre (hole centre) is correlated to 120 and 220 °C TL peaks and F⁺ centre (electron centre), which acts as a recombination centre is correlated to 120, 220 and 340 °C.     

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.     

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract

The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F⁺- centres in pure Al₂O₃ are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.     

Thermal dependence of impurity induced fluorescence in the system RbMgxMn1-xF3

The absorption spectrum of RbMnF₃ and the excitation spectra of the system RbMg_xMn_1-xF₃ at 10 K as well as the fluorescence spectra and lifetimes of Mn²⁺ in the systems RbMg_xMn_1-xF₃ and KMg_xMn_1-xF₃ in the region 10–300 K were measured. The lifetime and fluorescence temperature dependence suggest that the origin of the fluorescence occurs at Mn²⁺ sites slightly perturbed by impurity ions and that a non-radiative energy transfer mechanism is responsible for the observed thermal quenching. By using different Mn²⁺ concentrations in the above systems the dependence of the energy transfer on the Mn²⁺ concentration is shown. Finally, a preliminary observation on laser stimulated Mn²⁺ luminescence in the system RbMg_xMn_1-xF₃ is reported.     

Emission spectra of iron doped ZnSxSe1-x solid solutions

The photoluminescence of ZnS_xSe_1?x:Fe has been investigated in the visible and near infrared (λ ? 1.2 μm) region. The doping dependence shows, that the incorporation of iron in ZnS produces, in addition to the well-known red luminescence (λ_max = 660 nm), another emission band at 980 nm, which was hitherto unknown for ZnS: Fe. The energetic position of this new band is proved to be independent of composition of the ZnS_xSe_1?x solid solution. Therefore, it is interpreted as an internal transition in iron centres. On the other hand, the red Fe-band shifts to lower energy with increasing selenium concentration. This result shows clearly, that the red Fe-band of zincsulfide cannot be interpreted by an internal transition in iron centres [3], but favors the donor-acceptor model, which was already suggested by other authors [4].     

Luminescence of F + and F centers in YAlO3

In YAlO₃ crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F ⁺ centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F ⁺ and F centers in oxides of the Al₂O₃-Y₂O₃ system is analyzed on the example of the compounds α-Al₂O₃, YAlO₃, and Y₃Al₅O₁₂. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.     

Blue long-lasting phosphorescence of Ti-doped BaZrO3 perovskites

Long-lasting phosphorescence was observed from Ti-doped BaZrO₃ perovskite synthesized by a solid-state reaction. The phosphorescence color is blue and the phosphorescence can still be seen with the naked eye in the dark for 30 min or more after stopping UV irradiation. By the measurements of electron spin resonance (ESR) spectra, it was confirmed that F⁺ center and Zr³⁺ exist in non-luminescent undoped BaZrO₃ and their concentrations decreased with the additive amount of TiO₂. It is expected that the luminescence center is F_A center composed of Ti³⁺ and F⁺ center.     

The influence of nickel concentration on photoluminescence spectra of ZnSe:Ni samples

Photoluminescence (PL) spectra of ZnSe samples grown by iodine transport method and doped with Ni during the growth process and grown from the melt and doped with Ni by diffusion from a gas phase are investigated in visible and infrared (IR) ranges at 77 and 300 K. It is shown that under sub-band excitation, the PL spectra for the undoped samples of both types are similar and may be attributed to self-activated luminescence with participation of iodine- and V_Zn-based centres. The PL study for ZnSe samples doped with Ni carried out at various Ni concentrations, temperatures, and excitation intensities gives reason to ascribe the observed bands in near-IR and IR ranges to ³T₂(F)→³T₁(F) and ³T₁(P)→³T₁(F) intracentre transitions within the Ni_Zn²⁺(d⁸) ion.     

Doubly doped BaY2F8:Er,Nd VUV scintillator

Doubly doped BaY₂F₈:Er,Nd scintillation crystals were grown by modified micro-pulling-down method. The Er co-doping was chosen to enhance the energy transfer from the host lattice to the Nd³⁺ luminescence center via the 5d-levels of Er³⁺, which can be enabled by the overlap of Er³⁺ 5d-4f emission spectrum with the Nd³⁺ 4f-5d absorption. The energy transfer was clearly evidenced in the BaY₂F₈:Er,Nd. The processes are complicated by energy migration to killer centres and/or cross-relaxation processes. The luminescence and energy transfer mechanism are discussed.