FA Centers Research Articles

The origin of the 500 nm luminescence band related to oxygen vacancies in ZrO2

In this paper, ion-beam-induced luminescence (IBIL) spectra of raw ZrO2 irradiated with 2 MeV H+ ions and photoluminescence (PL) spectra of annealed (200, 500 and 800 °C in air, nitrogen or oxygen) ZrO2 were investigated. The luminescence intensity decreased with increasing fluence, which indicated that the concentration of luminescence centers decreased during irradiation. The PL spectral results show that, with increasing partial pressure of oxygen during annealing, the intensity of luminescence decreases due to the decrease in oxygen vacancies. First-principles calculations were used to study the formation energy and transition level for oxygen vacancies (Vo), Ti-substituted Zr (Ti) and Ti adjacent to the nearest Vo (Ti-Vo) complex. The binding energies of Ti3+ and Vo + are 2.10 eV, which indicates that oxygen vacancies benefit the formation of Ti3+. The calculated configuration coordinate diagram and IBIL results indicated that Vo is not the luminescence origin. A new light-emitting structure of the Ti-Vo complex (FA center) structure was proposed based on our calculations and previous studies. The calculated results of the luminescence model match the PL and IBIL spectra remarkably. The emission band of ZrO2 at approximately 500 nm was assigned to the FA center or Ti3+ (the eg to t2g transition) adjacent to Vo+.

Photochromism and emission-color change in Ba3MgSi2O8-based phosphors

Eu2+-doped Ba3MgSi2O8 phosphor, prepared by high-temperature solid state reaction in a reducing atmosphere, shows a blue emission under ultraviolet light excitation. When this phosphor is exposed to Xe-lamp radiation, the colorless powder turns orange, and the emission color changes to purple. The Xe-lamp induced orange color is successfully bleached by heat treatment at 350°C for 30min. Then, the emission color also returned to the original blue. The observed photochromic behaviors can be explained by electron transition in Eu2+-associated FA centers.

Spin Relaxation Processes via Tunneling Effect of the Li+ Ion in the FA (Li) Centers in Alkali Halide Crystals

Spin Relaxation Processes via Tunneling Effect of the Li+ Ion in the FA (Li) Centers in Alkali Halide Crystals

Vibronic relaxation processes of the FA center in RbCl:Li

The vibronic relaxation process of FA centers in RbCl:Li, which have Type I emission at low temperatures and Type II emission at high temperatures, has been studied at 10 and 77K by measuring the resonant secondary radiation (RSR) spectrum from optically excited FA centers with pico-second time-resolved spectroscopy. This study has been prompted by the quest to improve our knowledge on the de-excitation process in the FA centers. The adiabatic potential energy curves are deduced from a theoretical analysis of the measured RSR spectra and compared with those already known. It is shown that Type II relaxation observed at 77K occurs after Type I relaxation, supporting in turn a model proposed by Baldacchini et al. based on photoluminescence measurements.

Ｔｉを添加したペロブスカイト型酸化物ＢａＺｒＯ３の青色長残光特性

We report on the long-lasting phosphorescence in Ti-doped BaZrO3 perovskite-type crystals (synthesized by a solid-state reaction method). The phosphorescence color is blue and the phosphorescence can still be seen with the naked eye for 30 minutes or more in the dark after stopping UV light. By the measurement of electron spin resonance (ESR) spectra, it was confirmed that the defects of F+ center and Zr3+ exist in the Ti: BaZrO3 and their concentrations decrease with the additive amount of TiO2. It is expected that the luminescence center is FA center composed of Ti3+ and F+ center.

Vibronic relaxation processes of FA centers in KCl:Li

The vibronic relaxation process of FA centers in KCl:Li has been studied for the first time by measuring the hot luminescence spectrum from optically excited FA centers with time-resolved spectroscopy. It is shown that the FA center in KCl:Li has a relaxation process belonging to a Type I center relaxation behavior from its absorption at ∼670nm up to ∼840nm, although it is classified as a Type II center because of the large Stokes-shift of its emission band at ∼2.7μm. The relaxation after FA2 band excitation undergoes an anticrossing process due to the vibronic mixing effect of 2s and 2px,(y) states, similarly to that of F centers in KCl. For FA1 band excitation, the relaxation proceeds along the adiabatic potential of 2pz state, at least within the energy range investigated here. From the data obtained in the present study, the adiabatic potential energy curves of 2s and 2p states have been tentatively deduced.

The Measurements of Photostimulated Luminescence in Alkali Doped BaFBr:Eu2☎ at Liquid Nitrogen Temperature

Considerable increase of the photostimulated luminescence (PSL) intensity and red shift of the excitation spectrum was obtained by alkali doping of BaFBr:Eu2☎ crystals [1]. The band of the FA(Br−) centers about 0.1 eV shifted to low energy side against the “normal” F(Br−) centers. The FA(Br−) centers are destroyed after heating to 330 K.

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

By means of magnetic circular dichroism of the optical absorption and photostimulated luminescence (PSL) the X-ray induced formation of FA 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 FA 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 FA centres. However, up to 15% of the simultaneously generated F centres could be converted into the FA species by appropriate bleaching with 633 nm light into the F centre absorption band.

Photostimulation Redshift for BaFBr:Eu2+ with Alkali Doping

The X ray storage phosphor BaFBr:Eu2+ doped with Na+ gives a photostimulability shifted to lower photon energy. The photostimulation luminescence intensity considerably increases compared to BaFBr:Eu2+ upon room temperature X irradiation. This red shift of the photostimulation spectrum is caused by formation of FA (Na+) centres on the Br- sub-lattice as electron traps.

Thermally activated processes in the relaxed excited states of aggregate color centers in KF: Na

Optical absorption and photoluminescence spectra of KF crystals, doped with small concentrations of Na impurities and additively colored, are measured in order to determine thermally activated processes between different excited states of the FA centers. The mechanism of transformation of FA centers from type I (at low temperature) to type II (at higher temperatures) is fully confirmed by the anticorrelated temperature dependences of the intensities of the two FA(Na) luminescences peaking at 900 and 2150 nm.

Targeted mutations in the psaC gene of Chlamydomonas reinhardtii: preferential reduction of FB at low temperature is not accompanied by altered electron flow from photosystem I to ferredoxin.

The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit. To gain new insights into the electron transfer mechanisms through PsaC, we have generated three mutant strains of the alga Chlamydomonas reinhardtii in which two positively charged residues, K52 and R53, near the FA center have been altered in different ways. The mutations K52S/R53D and K52P/R53D lead to a strong destabilization of PSI. The third mutant K52S/R53A accumulates PSI to 30% of wild-type levels and shares the same residues between two of the cysteine ligands of FA as the PsaC homologue in the green sulfur bacterium Chlorobium limicola, in which FB has a higher redox potential than FA [Nitschke, W., Feiler, U., & Rutherford, A. W. (1990) Biochemistry 29, 3834-3842]. Low-temperature electron paramagnetic resonance (EPR) studies reveal that, in contrast to wild type, FB is preferentially photoreduced in this mutant, as was also observed for C. limicola. The preferential photoreduction of FB could be due to changes in the redox potential of FA and/or to slight structural modifications of the PsaC subunit. However, room temperature optical measurements show that stable charge separation still occurs and, surprisingly, that electron transfer from PSI to ferredoxin proceeds at normal rates in the mutant. As C. limicola, the K52S/R53A and K52S/R53D C. reinhardtii mutants are photosensitive when grown aerobically, but can grow photoautotrophically under anaerobic conditions.

Photostimulation redshift for nonstoichiometric Ba1−xSrxFBr:Eu2+

The oxygen-free x-ray storage phosphor BaFBr, doped with Sr2+ and having Br− vacancies due to fluorine excess, is shown to have a high photostimulability shifted appreciably to lower photon energy compared to nondoped BaFBr because of the predominant generation of FA(Sr2+) centers on the Br− sublattice upon room-temperature x irradiation. The FA(Br−,Sr2+) centers are less stable than unperturbed F(Br−) centers. Therefore, the redshift of the photostimulability disappears upon thermal activation above room temperature. From the failure to observe an F center infrared emission, it is concluded that F and FA centers are spatially correlated to hole centers of yet unknown nature.

Electron traps in Ca2+- or Sr2+-doped BaFBr:Eu2+ x-ray storage phosphors

The x-ray storage phosphor BaFBr:Eu2+ may be made sensitive to stimulation further into the infrared by doping with Ca2+ or Sr2+. This enables the use of light-emitting diodes instead of gas lasers for photostimulation. It is shown by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and optically detected EPR and ENDOR that upon x irradiation FA(Br−, Ca2+) centers are formed as photostimulable electron traps in Ca2+-doped BaFBr, in which one of the two nearest Ba2+ neighbors along the c axis is replaced by Ca2+. The FA(Br−, Ca2+) center has tetragonal symmetry as has the F(Br−) center. The optical absorption band of those FA(Br−, Ca2+) centers is red shifted. No FA(Br−, Ca2+) centers have been detected upon additive coloration only. The red shift of the photostimulation spectrum is caused by formation of the FA(Br−, Ca2+, or Sr2+) centers as electron traps.

Active photosynthesis in cyanobacterial mutants with directed modifications in the ligands for two iron-sulfur clusters on the PsaC protein of photosystem I.

The PsaC protein of the Photosystem I (PSI) complex in thylakoid membranes coordinates two [4Fe-4S] clusters, FA and FB. Although it is known that PsaC participates in electron transfer to ferredoxin, the pathway of electrons through this protein is unknown. To elucidate the roles of FA and FB, we created two site-directed mutant strains of the cyanobacterium Anabaena variabilis ATCC 29413. In one mutant, cysteine 13, a ligand for FB was replaced by an aspartic acid (C13D); in the other mutant, cysteine 50, a ligand for FA was modified similarly (C50D). Low-temperature electron paramagnetic resonance studies demonstrated that the C50D mutant has a normal FB center and a modified FA center. In contrast, the C13D strain has normal FA, but failed to reveal any signal from FB. Room-temperature optical studies showed that C13D has only one functional electron acceptor in PsaC, whereas two such acceptors are functional in the C50D and wild-type strains. Although both mutants grow under photoautotrophic conditions, the rate of PSI-mediated electron transfer in C13D under low light levels is about half that of C50D or wild type. These data show that (i) FB is not essential for the assembly of the PsaC protein in PSI and (ii) FB is not absolutely required for electron transfer from the PSI reaction center to ferredoxin.

The experimental observation of the potential barrier for self-trapped exciton decay into F-H pair in KCl-Na crystals

Abstract The optical absorption induced by the electron pulse irradiation of Na+ doped KCl has been measured. Transient optical absorption band of FA centers was observed at 80 K (LNT). The temperature dependence of FA center formation was studied. It is proposed that the obtained activation energy originates from the potential barrier between the STE perturbed by the cation impurity and the nearest neighbour FA-H pair. The mechanism of the suppression of the defect formation by the monovalent cation impurity in alkali halide crystals is discussed.

A light induced configurational change of FA centres in Li doped KCl-KBr crystals

Abstract Spectroscopic properties of FA centres in Li doped KCl-KBr mixed crystals were studied. At low temperature light induced spectral shifts, for the FA1 hand towards lower energy and for the FA2 band towards higher energy, were observed. The shifts are proposed to be due to a configurational change where the electron occupied vacancy finds a new location in relation to the neighbouring chlorine and bromine ions. The recovery to the original configuration, obtained in the F → FA conversion, is a temperature activated process.

Optical Absorption, Fluorescence, and Thermally Stimulated Emission on CaF2 : Dy: Pb Single Crystals

AbstractThe optical absorption (OA) spectra of unirradiated crystals of CaF2 doped with 0.1% Dy (by mol%) and 0.1% Pb, hereafter called crystal I, and doped with 0.2% Dy and 0.1% Pb, called crystal II, indicate the presence of a small amount of dysprosium in their divalent state (Dy2+). There is an increasing absorption towards 6.5 eV. On y‐irradiation the concentration of Dy2+ ions is found to increase. Fluorescence of these crystals shows characteristic Dy3+ emission at 1.62, 1.85, 2.16, and 2.85 eV (4F9/2→6H9/2), 6H11/2, 6H13/2, 6H15/2. In thermoluminescence (TL) glow after γ‐irradiation at room temperature crystal I shows six peaks in the temperature range (300 to 700 K) studied. Dy3+ is found to be the major luminescent centre under all these glow peaks. In addition to the Dy3+ emission bands there are two bands in the UV region at 4.94 and 5.93 eV in TL emission. For higher concentrations of lead the results are somewhat different. Crystal III (with 0.01% Dy and 0.5% Pb) and crystal IV (with 0.02% Dy and 0.5% Pb) show a strong absorption band around 6.4 eV. On irradiation prominent absorption bands are observed around 2.04 and 3.21 eV. These are attributed to an F centre associated with the rare‐earth ion in its divalent state (Dy2+ –F centre complex), akin to the FA centre in alkali halides. It is also found that the lead ions (Pb2+) when excited, transfer their energy to nearby Dy3+ ions, leading to the characteristic emission of Dy3+ ions.

Thermally activated reorientation of FA(II) centers in RbCl:Li+ crystals

Thermally activated reorientation of F_A(II) centers in RbCl:Li⁺ crystals

On the off-centre angle and the refractive index difference due to the birefringence of FA (II) centres in KCl:Li crystals

Based on the dichroism of the FA (II) KCl:Li centres the off-centre angle of the Li+ ion was measured to be 13.5+or-1 degrees at 26 K. Furthermore, the birefringence of the centres was verified and the difference between the refractive indexes relating to the two preferred directions was directly derived from the absorption spectra measured with polarized wave.

Energy transfer between F-type defects in CaO studied by optical-microwave double-resonance spectroscopy

Cascade energy transfer between photo-excited F+, F+A, F and FA point defects in CaO is reported utilizing highly selective optical-microwave double-resonance methods. Time-resolved MIDP experiments show a shortening of the lifetime of the Tx, y spin sublevels in the photo-excited triplet state of F defects in CaO: Mg. The lifetime shortening is attributed to the effect of energy transfer to the FA centers in this host crystal. A rate constant of 53 s−1 characterizes the transfer process at T = 1.2 K.