Search

A study is performed of optical spectra (absorption, excitation, luminescence, energy yield) associated with the upconversion of infrared excitation of 980 nm into visible radiation in crystals of alkaline–earth fluorides CaF2, SrF2, and BaF2, co-doped with YbF3 (0.01–10 mol %) and HoF3 (0.01–0.3 mol %). In the 0.1–10 W cm−2 range of excitation power densities, the intensity of the upconversion bands at 542, 650, and 752 nm grows as the square of the power. A quadratic dependence of the intensity of upconversion on the concentration of Yb3+ (0.03–3 mol %) and a weak dependence on the Ho3+ concentration (0.01–0.3 mol %) with a maximum at 0.1–0.15 mol % are observed. It is assumed that the upconversion is due to consecutive energy transfer from two closely spaced excited ytterbium ions to the holmium ion.

The dielectric spectra of LaF3 crystals activated by rare-earth ions Eu2+, Sm2+, and Yb2+, gradually cooled from 300 to 240 K, were investigated. Obtained experimental data were compared with experimental data obtained previously by other authors. The reorientation activation energies for RE2+ dipoles in LaF3 are 0.49, 0.55, 0.56, and 0.58 eV for Ba2+, Sm2+, Eu2+, and Yb2+, respectively. A reorientation of fluorine vacancy is observed, across the F1 positions.

The results from a theoretical study of the electron structure of an impurity rare-earth Sm2+ defect in a LaF3 crystal are presented. The electron energy levels of the rare-earth impurity defect and the transitions between them are studied using the multiconfigurational CASSCF/CASPT2 method. The absorption spectrum obtained during the calculations is consistent with the experimental data. Based on our model, we can state definitively that a vacancy on an anion sublattice serves as a charge compensator for a divalent ion.

The mechanism of coloration of alkaline-earth fluoride crystals CaF2, SrF2, and BaF2 in calcium vapors in an autoclave with a cold zone is studied. It was found that the pressure in the autoclave upon constant evacuation by a vacuum pump within the temperature range of 500–800°C increases due to evaporation of metal calcium. In addition to the optical-absorption bands of color centers in the additively colored undoped crystals or to the bands of divalent ions in the crystals doped with rare-earth Sm, Yb, and Tm elements, there appear intense bands in the vacuum ultraviolet region at 7.7, 7.0, and 6.025 eV in CaF2, SrF2, and BaF2, respectively. These bands belong to the Ha - hydrogen centers. The formation of hydrogen centers is also confirmed by the appearance of the EPR signal of interstitial hydrogen atoms after X-ray irradiation of the additively colored crystals. Grinding of the outer edges of the colored crystals leads to a decrease in the hydrogen absorption-band intensity with depth to complete disappearance. The rate of hydrogen penetration inside the crystal is lower than the corresponding rate of color centers (anion vacancies) by a factor of tens. The visible color density of the outer regions of the hydrogen-containing crystals is several times lower than that of the inner region due to the competition between the color centers and hydrogen centers.

Optical spectra (absorption, emission, excitation, decay) and dielectric relaxation were measured for divalent europium and partially for ytterbium in lanthanum fluoride crystals. Optical absorption of LaF 3 –Eu 2+ contains not only asymmetric weakly structured band at 245 nm but also less intense bands at 330, 380 nm. Broadband Eu 2+ emission at 600 nm appeared below 80 K with decay time 2.2 μs at 7.5 K. Emission at 600 nm is attributed to so-called anomalous luminescence. Bulk conductivity is directly proportional to absorption coefficient of Eu 2+ bands. Dielectric relaxation peak of LaF 3 –EuF 3 is attributed to rotation of dipoles Eu 2+ -anion vacancy. The long-wavelength absorption at 300–400 nm region are assigned to transitions from 4f 7 Eu 2+ ground state to states of neighbouring fluorine vacancy.

We report data on the luminescence spectra associated with photochromic centers in X-ray irradiated calcium fluoride crystals doped with Lu ions. Irradiation in low energy photochromic centers absorption band excites emission, which can be identify with transitions into photochromic centers. Ab initio calculation of absorption spectrum of photochromic center agrees rather well with experimental data.

In this paper we present theoretical study of Ce 2+ impurity centres in alkaline earth fluoride crystals (CaF 2 , SrF 2 ). Only cubic configurations of centres were considered. Electronic levels and related properties were studied using CASSCF/CASPT2 approach within embedded-cluster formalism including scalar relativistic corrections and spin-orbital interaction. Calculated absorption spectra for Ce 2+ in CaF 2 and SrF 2 are in good agreement with experimental data. For both crystals the ground state of Ce 2+ ion has predominantly 4f 1 5d 1 singlet character.

The UV absorption spectra of F 3 − molecular ions in LaF3, SrF2, CaF2, and BaF2 crystals doped with rare-earth elements are studied. Comparison of radiation-colored and additively colored crystals reveals the absorption bands of F 3 − hole centers in the region near 6 eV. Nonempirical calculations of optical transitions agree well with experimental results.

The optical spectra and electrical conductivity of LaF3 crystals containing Sm2+ ions are studied. Groups of emission lines at 560–620, 650–690, and 680–770 nm at 7.9 K are produced by transitions from states 5 D 2, 5 D 1, and 5 D 0 to states 7 F j of Sm2+ ions. Radiative transitions from state 5 D 0 are characterized by a decay time of 8.9 ms and temperature quenching in the range of 70–160 K. A linear dependence of the crystals’ electrical conductivity on the magnitude of absorption bands of Sm2+ ions is observed, due the presence of anionic vacancies that compensate for the low charge of Sm2+ ions. The long-wavelength absorption band at 600 nm that is not observed in the excitation spectra is produced by the transition from 4f Sm2+ to the 1s level of anion vacancies. Absorption bands with wavelengths below 520 nm are due to 4f-5d transitions in the Sm2+ ions.

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF 2 , SrF 2 , BaF 2 ) crystals doped with Nd 3+ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4f n –4f n−1 –5 d 1 transitions of Nd 2+ . The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd 2+ → Nd 3+ ) occurs under heating to 570 K in CaF 2 , 520 K in SrF 2 and 470 K in BaF 2 .

We have studied the absorption spectra of radiation-induced divalent cerium and praseodymium ions in crystals of alkaline-earth fluorides. Using ab initio quantum-mechanical methods, we have calculated absorption spectra of divalent praseodymium ions in CaF2 crystals for the first time. The theoretical spectrum agrees rather well with the experimentally registered spectra.

In this paper we present the results of an ab initio calculations of df-absorption spectra for alkaline earth fluoride matrix doped with Pr3+ and Ce3+ ions. The density-functional theory (DFT) calculations have been performed in embedded-cluster approach. All our results were obtained with taking into account the lattice relaxation in the presence of trivalent impurity ion. The calculation have been made both for cubic and tetragonal centers. For the tetragonal centers we estimate the crystal field splitting of eg level.

This paper reports the absorption spectra of photochromic centers in CaF2 and SrF2 crystals doped with Ce3+ and Gd3+ impurities and thermal decay of the centers in temperature range 80-500 K. The ionized photochromic color centers are generated in crystals under low-temperature X-rays irradiation. These centers are transformed into photochromic color centers upon heating of crystals. All color centers decay at temperature about 500 K.

Emission and excitation spectra of CaF2, SrF2, BaF2 doped with Nd3+, Sm3+, Ho3+, Er3+, Tm3+ excited by vacuum ultraviolet photons or by x-rays were studied. Decay times of the Nd3+ emission near 180 nm at 7 K were 17.7, 15.3, 12.3 ns in CaF2 , SrF2 , BaF2 respectively. 4fn-1 5d1-4fn luminescence bands were found at 170-200 nm for Sm3+ ions and at 140-190 nm for Ho3+. No excitation of 5d-4f emission of Nd3+ and Tm3+ was observed in the region of band-to-band transitions below 127 nm (BaF2), 120 nm (SrF2) and 115 nm (CaF2). Emission spectra of Nd and Er doped alkaline-earth fluoride crystals under x-ray excitation were studied with concentration of impurity from 0.01 to 10 molar %. For the Nd3+ and Er3+ 5d-4f emissions in SrF2 the light yields were estimated as 30-40 ph/Mev.

The paper is devoted to investigation of BaF2, SrF2, CaF2 doped with different concentrations of praseodymium or cerium as scintillator for well-logging. Temperature dependences of light yield at the higher then room temperatures, pulsed-height and emission spectra of fluorides were measured.

Ionization radiation is shown to reduce impurity ions to the univalent state in strontium and calcium fluoride crystals doped by divalent cadmium and zinc ions. In this case, a univalent ion is surrounded by eight equivalent fluorine ions and exhibits cubic symmetry Oh. At room temperature, the symmetry of the center is revealed to be sequentially lowered to C3v and then to C2v owing to the addition to the nearest environment of the impurity univalent ion of one or two anion vacancies, respectively, which are intrinsic defects not forming in undoped strontium and calcium fluoride crystals. Stable intrinsic defects are assumed to form through the separation of anion vacancy-interstitial fluorine ion pairs in the electric field induced by the reduced impurity ions. This electric field lowers the energy barrier to thermal separation of charged intrinsic defects.

Absorption, excitation in vacuum ultraviolet region, emission and decay of emission of alkaline-earth crystals (CaF 2 , SrF 2 , BaF 2 ) doped by different concentrations of YF 3 or LaF 3 were studied. New emission bands in the region 3-6 eV and excitation bands just below exciton edges were observed. Unempirical Hartree-Fock and density functional calculations of alkaline-earth crystal clusters containing Y or La impurity were performed. We concluded that observed optical bands belong to transitions between interstitial fluorine and trivalent metal parts of pairs Me 3+ -F - i .

We study the aggregation of oxygen dipoles well dispersed in a CaF2 crystal upon annealing at temperatures ranging from 370 to 420 K. The concentration of oxygen dipoles is monitored by measuring the intensity of the ionic thermocurrent peak as well as by absorption and luminescence spectroscopies. Results from three methods agree within experimental error and yield an activation energy of (1.2±0.1) eV for the diffusion of isolated oxygen centres in the crystal.

We have studied the absorption spectra of x-ray irradiation-induced Ce2+ and Pr2+ ions in crystals of alkaline-earth fluorides. We have calculated absorption spectra of divalent praseodymium ions in SrF2 crystals doped with Pr2+ for the first time. The calculated spectra agree rather well with the experimental data. In crystals containing induced Ce2+ ions we have found strong electron-phonon coupling. In BaF2, we do not observe bands corresponded to divalent Ce or Pr ions., Comment: 14 pages, 5 figures, 1 table

Strong absorption in the vacuum ultraviolet region near the exciton edge was observed in Cd doped CaF 2 , SrF 2 and BaF 2 crystals. The Cd + + absorption band at 8.6 eV was observed in low doped BaF 2 crystals. In SrF 2 and CaF 2 the maximum of Cd + + band cannot be resolved due to stronger overlapping with exciton bands. The calculations of geometry of Cd + + and Cd + centres and Cd- related electron transitions were done using ab - initio Hartree-Fock and density functional methods. The experimental and calculated results lead us to the conclusion that strong Cd + + absorption bands belong to allowed transitions from the nearest fluorides to the Cd + + ion. Lattice relaxation around Cd + centres, created by x-irradiation, and optical transitions of these centres were also calculated. The calculated energies are in good agreement with experimental ones.

Oxygen-vacancy dipoles and dimers in CaF2 crystals have been studied ab initio at DFT level and with the shell model using pair potentials. The calculated dipole reorientation barrier is 0.64 eV and the activation energy for diffusion of the dipoles is 1.61 eV. Optical absorption of O 2--VA dipole have been calculated with TD DFT and identified with experimental absorption bands, which appeared to have complex structure. The photodissociation mechanism of the dipole is discussed. Several configurations of the dimer (O2--VA)2 were calculated. The association energy for the most favourable one is 0.48 eV. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Defect formation in BaF2 single crystals doped with cadmium is investigated by optical absorption, electron paramagnetic resonance (EPR) and thermoluminescence. An optical absorption band observed in X-irradiated at 295 K crystals CaF2 (at 3.8 eV), SrF2 (at 3.95 eV) and BaF2 (at 4.08 eV) is assigned to absorption of Cd+-ions. In addition, two type hole centers: pure Vk and perturbed Vk-centers, are observed in BaF2–1%CdF2 crystals X-irradiated at 77 K.

Optical absorption spectra in the vacuum ultraviolet region and high resolution emission and absorption spectra in the ultraviolet region for cubic and tetragonal Ce3+ centres in SrF2 crystals were measured. Cubic centres are characterized by a 33 250 cm−1 zero-phonon line and a sharp phonon line spaced from the zero-phonon line by 410 cm−1. The decay times of cubic and tetragonal centre luminescences are 32 ns. Comparison of cubic and tetragonal spectra allows us to establish the influence of charge compensating interstitial fluorine ions on f and d levels of Ce3+ ions. Non-empirical Hartree–Fock calculations for Ce states in molecular clusters of alkaline-earth fluorides were performed. To make the calculated transitions coincide with the experimental ones one needs to take into account a shift of Ce3+ and interstitial F− towards each other by 10% of the initial distance.

A study of the luminescence characteristics of crystalline lithium triborate LiB3O5 (LBO) is reported. Investigation of the excitation and photoluminescence spectra of nominally pure, oriented LBO crystals within broad spectral (1.2–10.5 eV) and temperature (8–500 K) regions, complemented by optical spectroscopy at the long-wavelength fundamental-absorption edge, has revealed that the broad-band LBO luminescence in the 3.5–4.5-eV region is efficiently excited by photons having energies above 7.5 eV in recombination processes and under corpuscular or x-ray irradiation. The totality of the experimental data obtained permitted a conclusion that the LBO luminescence has an intrinsic nature and that it originates from radiative decay of relaxed electronic excitations.

Results of absolute light output measurements on strontium and barium fluoride doped with PrF$_3$ and CeF$_3$ are presented and compared with scintillators having well-known light output (NaI-Tl, CsI-Tl, BGO). For pure SrF$_2$ crystal we obtain a value of about 28600 photons/MeV., 5 pages, 2 figures

Absorption, excitation and emission spectra of cadmium fluoride crystals doped with rare-earth ions were investigated. In contrast to alkaline-earth fluorides the absorption spectra due to 4f - 5d transitions of Ce$^{3+}$, Pr$^{3+}$ and Tb$^{3+}$ ions are broadened. No 5d-4f emissions were observed. These prove that 5d(e$_g$) levels of rare earth ions lie in conduction band of CdF$_2$ crystal. Emission spectra of Tb$^{3+}$ show the group of 4f-4f $^5$D$_4$-$^7$F$_j$ lines in contrast to other alkaline-earth fluorides where emission due to $^5$D$_3$-$^7$F$_j$ transitions is also observed. The absence of the emission is due to position of $^5$D$_3$ level within condution band. Among the all measured crystals doped with impurity ions (Pr, Nd, Eu, Ho, Tb, Tm, Yb, Ga, In or Mn), the CdF$_2$ doped with Pr$^{3+}$, Tb$^{3+}$, or Mn$^{2+}$ ions have the highest light outputs under x-ray excitation., Comment: presented on conference SCINT2013

Oxygen impurities enter BaFCl and BaFBr crystals as oxygen-vacancy centers. As in other ionic crystals, these defects in the barium fluorohalide crystals have several absorption bands and one photoluminescence band. The defects also are involved in aggregation and photochemical reactions. We have found two kinds of oxygen-vacancy centers in barium fluorohalide crystals and denoted them as centers of types I and II. On the basis of the growth conditions, absorption spectra and other data one may conclude that centers of the first type consist of oxygen substituting at F− sites and neighboring Cl− or Br− vacancies. Oxygen-vacancy centers of the second type seem to be completely on either the Cl or the Br sublattice.

Two decay components of luminescence of oxygen-vacancy centres were observed in alkaline-earth fluoride and barium fluorohalide crystals at 17-300 K. We denoted them as slow and very slow components. Very slow components are observed in all crystals except for type II oxygen centres in BaFBr. Above 100-150 K the integrated intensity of very slow components decreases in parallel with the decay time with thermal energy 0.1-0.2 eV. The integrated intensity of the slow component is not decreased from 17 to 300 K, while the decay time decreases from several hundred microseconds to several microseconds. This behaviour is explained by thermal interaction between three closed excited levels with different radiative lifetimes. We tentatively assume that these three levels belong to different oxygen-vacancy centre configurations in the lowest excited state.

The formation of radiation defects in calcium and strontium fluoride single crystals doped with cadmium or zinc has been investigated by luminescence and absorption spectroscopy, as well as by electron spin resonance spectroscopy. It was found that x-irradiation could convert divalent impurity ions located at essentially cubic sites into the univalent state. Three types of Cd(+) or Zn(+) centers differing by local environment with point symmetries O(h), C(3v) and C(2v) are identified in the crystals. The formation of the last two results from the interaction between reduced impurities in the cubic environment and anion vacancies. The latter are intrinsic radiation defects and are not created by x-irradiation in undoped crystals. We also discuss the possible implications of the electric field of the charge impurity defects on separation of the intrinsic radiation defects in these crystals.

The spatial structure of FH(O2−) centers in CaF2 is studied by optical dichroism and ESR methods. The dichroism of absorption is induced by light in both FH bands in additively and X-ray colored samples. The results are correlated with the C2 crystal axes of FH defects. The ESR spectrum of FH centers is rather similar to that of pure F centers. Both dichroism and ESR results prove that an oxygen ion occupies a (110) anion site around the F center. [Russian Text Ignored].

The absorption, emission, excitation spectra, and polarized luminescence of titanium centers in lithium and sodium fluorides are studied. The 2.42 and 3.1 eV emissions in LiF:TiO2 and 2.2 and 2.95 eV emissions in NaF:TiO2:MgF2 are related to the electronic transitions of titanium centers. The low energy emissions at relatively low temperatures have the same azimuthal dependence of the polarisation degree as the emission of oscillators oriented along the C2 crystal axis. The high energy emissions are polarized along the C3 crystal axis. The high temperature stability of the polarization degree shows that the luminescence centers have point defects near the titanium ion. The efficiency of transformation of titanium luminescence centers by X-irradiation grows above 200 K. This points out that the cation vacancies are part of luminescence centers. A model of titanium luminescence centers in lithium and sodium fluorides is suggested. The center consists of a Ti4+ ion at cation site, an O2− ion at anion site, and two associated cation vacancies. [Russian Text Ignored].