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The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Нo, Er, Tm ions are investigated by thermal bleaching of the sample in the 300–700 K temperature range. X-ray irradiation creates absorption bands of inter-configuration 4fn-4fn−1-5 d1 transitions of divalent impurity ions and F3− and (F2−)ii hole centers. The thermal oxidation of the divalent RE ions have been observed. Two possible mechanism of thermal oxidation of divalent lanthanide ions in alkaline-earth fluorides are discussed.

Processes of photo-induced electron transfer from a bivalent acceptor lanthanide (Eu, Sm, and Yb) onto a trivalent donor lanthanide (Nd, Sm, Dy, Tm, and Yb) and the inverse thermally activated transfer are studied in barium fluoride crystals. At room temperature, photoinduced electron transfer is accompanied by oncoming displacement of the interstitial charge-compensating fluoride ion. On photoquenching at low temperatures, a bivalent donor lanthanide remains next to the interstitial fluoride ion, which causes a redshift of its 4f–5d absorption bands. The shift increases as the lanthanide size decreases (as in the series Nd, Sm, Dy, Tm, and Yb). A detailed analysis of the mechanisms of photo and thermal electron transfers between the different lanthanide centers in BaF2 crystals is provided.

Forward electron photo-transfer and reverse thermal transfer between divalent and trivalent heterogeneous lanthanides in barium fluoride crystals has been studied using optical absorption. In crystals activated by two heterogeneous lanthanides and grown in reducing conditions, one of the lanthanides becomes bivalent whereas the other lanthanide remains trivalent. Illumination of the crystal in the ultraviolet bands led to the transfer of electrons from divalent lanthalides (Eu, Yb, Sm) to trivalent ions (Ho, Nd, Dy, Tm, Sm, Yb). The thermal ionization energies to the conduction band of created lanthanides are determined from the thermal bleaching curves of absorption bands. The experimental energies are compared with the estimated energies of the Dorenbos model.

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 absorption of a nanocomposite of silver nanoparticles and arabinogalactan in the UV–Vis spectral range is due to the presence of end aldehyde groups in the arabinogalactan and plasmon vibrations in 0D nanosilver. The absorption spectrum of a fundamentally new nanocomposite of silver with arabinogalactan-g-polypyrrole block copolymer reveals additional long-wavelength overlapping absorption bands resulting from the longitudinal component of plasmon resonance in 1D nanosilver and polarons in polypyrrole.

The optical spectra and electric conductivity of LaF3 crystals doped with 0.01, 0.1, and 0.3 mol % YbF3, where Yb was partly or completely recharged to the divalent state, are studied. The long-wavelength absorption band of 370 nm is caused by electrons transitioning from state 4f 14 to the level of anion vacancies. The remaining bands at 300–190 nm are caused by 4f 14–5d 14f 13 transitions in Yb2+. The bulk electric conductivity and peaks of the dielectric losses of LaF3–Yb2+ crystals are caused by Yb2+–anion vacancy dipoles. The activation energy of the reorientation of Yb dipoles is 0.58 eV. The optical and dielectric properties of Yb2+ centers are compared to those of Sm2+ and Eu2+ centers studied earlier in LaF3 crystals.

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.

The absorption spectra of LaF3 crystals, both pure and doped with rare-earth fluorides (YF3, CeF3, NdF3, PrF3, SmF3, EuF3, GdF3, TbF3, DyF3, HoF3, ErF3, TmF3, YbF3, and LuF3) have been investigated. All these impurities can be separated into two groups with respect to the shape of the absorption spectra of irradiated crystals. The spectra of the crystals doped with Nd, Sm, Tm, and Yb exhibit, along with 200-nm hole band F3-, weak bands due to RE2+-anion vacancy centers. The spectra of LaF3 crystals with Y, Ce, Pr, Gd, Tb, Dy, Ho, Er, and Lu impurities exhibit, along with the hole-center bands (F3- at 200 nm and VkA at 320 nm), bands of comparable intensity, which can be attributed to RE3+–F centers. This conclusion is confirmed by preliminary quantum-chemical calculations and the estimation of the levels location in the energy-band diagram.

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 .

The structure of the absorption spectra of Ce3+, Pr3+, and Tb3+ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce3+, Pr3+, and Tb3+ ions in CdF2 can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm−1. The broadening is related to the autoionization of electron from the local 5d(eg) level to the energy-degenerate states of the conduction band of CdF2 crystal.

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 results of scintillation properties measurements of pure and Ce3+-doped strontium fluoride crystals are presented. We measure light output, scintillation decay time profile and temperature stability of light output. X-ray excited luminescence outputs corrected for spectral response of monochromator and photomultiplier for pure SrF2 and SrF2-0.3 mol.% Ce3+ are approximately 95% and 115% of NaI-Tl emission output, respectively. A photopeak with a 10% full width at half maximum is observed at approximately 84% the light output of a NaI-Tl crystal after correction for spectral response of photomultiplier, when sample 10x10 mm of pure SrF2 crystal is excited with 662 KeV photons. Corrected light output of SrF2-0.3 mol.% Ce3+ under 662 KeV photon excitation is found at approximately 64% the light output of the NaI-Tl crystal., Comment: 4 pages, 4 figures and 1 table

Time-resolved emission and excitation spectra as well as emission decay kinetics of CaF 2 , SrF 2 , BaF 2 doped with HoF 3 were investigated. Intensive emission bands near 168 nm, having long decay time, are caused by the spin-forbidden transitions from the 5d 1 4f 9 high-spin states to the ground 5 I 8 states of Ho 3+ ions. Weak spin allowed 5d 1 4f 9 (low-spin)-4f 10 emission band at 158 nm was observed only in CaF 2 –Ho crystals. Spin allowed and spin-forbidden excitation bands were observed near 166 and 155 nm, respectively, in all studied crystals. Fast component of spin-forbidden emissions due to multiphonon relaxation to low-lying 4f 10 Ho 3+ level also was observed for all crystals.

This Letter presents the results of measuring scintillation properties of pure SrF2 crystals and crystals activated by various concentrations of Ce3+ ions. The light yield of these materials is compared to that of the known scintillators NaI-Tl and CaF2-Eu2+. Strontium fluoride crystals activated with Ce3+ ions are found to be characterized by high light yield and to be promising materials for use in scintillation detectors employed for γ-ray well logging.

Using ab initio methods and taking into account the lattice relaxation and polarization caused by the occurrence of the core hole, we have studied theoretically the cross luminescence in barium fluoride crystals in terms of the embedded-cluster approach. Two schemes of modeling of the core hole have been performed—in the form of an additional point charge and in the form of the 5p state of the barium ion. Calculations have been done both by the Hartree-Fock method and by the density functional method. We have showed that the deformation of the lattice caused by the occurrence of the core hole leads to states localized on fluorine ions of the nearest environment splitting off from the valence band of the BaF2 crystal. The cross-luminescence bands at 5.7, 6.3, and 7.1 eV are caused by transitions from these localized states. We have also showed that the low-energy edge of the cross luminescence is formed by transitions from states that are localized on ions of the second coordination sphere.

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 vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF2, SrF2, BaF2) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.

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.

We present the results of ab initio calculations of self-trapped excitons (STE) in CaF2 and SrF2 crystals performed with BHHLYP density functional in embedded cluster approach. As a motivation for this theoretical study, we also report some previously unpublished experimental results on STE luminescence. They concern new luminescence bands of STE approximately 1 eV higher then the main band. The new bands have much lower intensities in pure crystals, however they remain almost unaffected by impurity doping while the main band is quenched by impurities. As possible candidates for excitonic configuration responsible for these high-energy bands, four possible configurations of the off-center STE are considered and rejected. For the on-center STE, i.e., VK + e, stable configuration has been found. The calculated luminescence energy of on-center STE is 6.57 eV in CaF2 and 6.31 eV in SrF2 crystal. On the basis of these energies (much higher than energies of new excitonic luminescence bands) and low stability of on-center STE, it is also rejected as a candidate for new bands. Finally, the hypothesis of electron self-trapping is considered. We have found electron self trapping in CaF2 and SrF2 crystals to be energetically favorable, however the value of energy gain lies within the possible calculation error. It was also found that there is a barrier about 0.3 eV for electron self-trapping. Self-trapped electron is a center with trigonal symmetry and can be considered as perturbed F-center. Calculated isotropic hyperfine couplings of self-trapped electron are given. On the basis of these results, we tentatively suggest that self-trapped electrons are the precursors of yet unknown excitonic configuration responsible for the new luminescence bands.

Absorption, emission, excitation spectra of CaF2, SrF2, BaF2 crystals doped by PrF3 in concentration ranging from 0.005% to few molar % were studied in 1.4-10 eV spectral region. Specific type of Pr centres possessing photon cascade emission (PCE active) was observed in all crystals. Concentration of centres monitored by excitation and emission spectra grows superlinearly with increasing of Pr concentration in SrF2 and BaF2 crystals. A new additional absorption band at 7.0 eV which coincided with excitation band of PCE-active centres was observed in CaF22 Pr crystals. The absorption coefficient of this band grows twice that coefficient of 5.7 eV band of single Pr centres.

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.

This paper reports on a study of cross-luminescence in barium fluoride crystals doped by a variety of impurities (K+, Cd2+, Y3+, Yb3+, La3+). It is shown that doping of the crystal with a trivalent impurity gives rise to the formation of an additional cross-luminescence band peaking at 7.5 eV, the intensity of this band increasing with increasing impurity concentration.

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4fn-4fn − 15d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5–2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C4v, Oh, and R aggregates. Excitation in the CT2 bands revealed luminescence of only C4v defects.

This paper presents both investigations of high-energy luminescence with maximum at 7.5 eV in BaF2:Y3+ and BaF2:Yb3+ crystals and theoretical calculations of cross-luminescence spectrum in crystals of barium fluoride with interstitial ions of fluorine. The dependencies of the 7.5 eV emission intensity as function of different impurities mol. concentration are presented. The intensity of this emission increases with concentration of rare-earth impurity, but in the case of doping by Yb3+ and Y3+ this intensity is less than that in the case of La3+ doping. This feature (as well as the nonlinear dependence) can be due to the creation of clusters, i.e., aggregation of impurity ions and F--interstitials. The intensity of the 7.5 eV luminescence in a BaF2:Y3+ crystal after annealing at 1000 K increased by three times but it remained less than in the case of La3+ doping. Theoretical calculations of the core hole electronic and spatial structure and cross-luminescence spectrum were performed in the [Ba6F12] cluster. This cluster contains interstitial ion of fluorine, which is surrounded by six ions of Ba2+. In the case of using Hartree-Fock method the calculated luminescence spectrum of BaF2 crystal with interstitial fluorine shows cross-luminescence band with main maximum at 5.2 eV and high-energy band at 11.7 eV. In the case of using TD DFT method the energies of both cross-luminescence transitions and of transition between interstitial ion and core levels are in satisfactory agreement with experimental data, being all overestimated by approximately the average value of 0.8 eV.

a b s t r a c t Absorption, emission and excitation spectra of CaF2, SrF2, BaF2 crystals doped by YbF3 or EuF3 impurities were studied in 1‐12 eV spectral region. The intensive absorption broad bands (denoted as CT1) were observed in all cases just below the 4f‐5d absorption region. Less intensive absorption bands, denoted as CT2, having energies 1.2‐1.5 eV lower than those of CT1, were observed in CaF2, SrF2 crystals doped by EuF3 or YbF3 impurities. High resolution emission spectra of Eu-doped CaF2 and SrF2 crystals excited into CT1 and CT2 bands were measured. Under excitation into CT1 bands, all present Eu sites (C4v, Oh and some aggregates) were observed in emission spectra. While under excitation into CT2 bands, only the emission of C4v sites was observed.

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

The emission and excitation spectra as well as decay of emissions of alkalineearth fluoride crystals doped with CdF2 were investigated in the 2–24 eV range at temperatures in the range 8–300 K. Emission bands at 4.2 and 3.5 eV, respectively, were found under excitation into the Cd absorption region in CaF2–Cd and SrF2–Cd crystals at low temperatures. Both emission bands have slow luminescence decay times of microsecond timescale. No Cd-related emission was found in BaF2–Cd crystals. The calculations of the geometrical configurations of excited triplet Cd 2+ centres and the Cd-related electron transitions were carried out by using the ab initio Hartree–Fock method. The results of experiments and calculations lead us to the conclusion that the observed Cd 2+ emission bands are due to the triplet–singlet transitions from the Cd s-states to the nearest fluorine ions. The calculated energies are in good agreement with experimentally observed ones. (Some figures in this article are in colour only in the electronic version)

Optical absorption excitation and emission of alkaline-earth fluoride crystals doped with impurities, which did not introduce a new absorption band up to 8 eV, were investigated. Comparison of emission of BaF 2 –LaF 3 , BaF 2 –KF and doubly doped BaF 2 –LaF 3 , KF shows that interstitial fluorines are not the main exciton suppressors as thought before.The two stages of exciton suppression on the plot of exciton intensity against impurity concentration were observed. The first stage has the interaction length near 130 A, while the interaction lengths of second stage are 10–30 A, depending on impurity.

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, and high resolution emission and absorption spectra in the ultraviolet region of cubic and tetragonal Ce3+ centres in SrF2 crystals were measured. Cubic centres are characterised by the zero phonon line at 33250 cm −1 and a sharp phonon line is spaced from the zero phonon line by 410 cm −1 . The decay times of the luminescence from the cubic or tetragonal centres are equal to 32 ns . A comparison of the cubic and tetragonal spectra allows us to determine the influence of the charge compensating interstitial fluorine ion on the f and d levels of the Ce3+ ions. Unrestricted Hartree–Fock calculations of Ce states in molecular clusters of alkaline-earth fluorides were performed. To make the calculated transitions coincide with the experimental one it is necessary to take into account the shift of the Ce3+ and interstitial F− ions toward each other by 10% of initial distance.

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.

Considerable increase of the photostimulated luminescence (PSL) intensity and red shift of the excitation spectrum was obtained by alkali doping of BaFBr:Eu 2+ crystals [1]. The band of the F A (Br - ) centers about 0.I cV shifted to low energy side against the normal F(Br - ) centers. The F A (Br - ) centers are destroyed after heating to 330K.

New emission bands were found in Ce3☎ doped SrF2 and BaF2 crystals under excitation into the charge transfer region. The bands at 4.85 eV in BaF2 and 4.35 eV SrF2 were found in all crystals with Ce-concentration from 0.001 to 1 mol.%, most prominent in 0.01% samples. Decay of luminescence was simple exponential in BaF2, decay time decreased from 1.3 ms at 65 K to 0.41 μs at 523 K. The decay curve in SrF2 shows t−0.3 dependence at room temperature. All experimental results were naturally explained by assumption that new emission belongs to radiative charge transfer recombination in Ce2☎—Fio pairs with different distances between them.

Exciton luminescence was investigated in La-doped BaF2 crystals. Two luminescence bands at 4.26 and 3.82 eV related to excitons have been observed. The excitation spectrum of the 4.26 eV band of BaF2–LaF3 was similar to that of the exciton emission band of pure BaF2 crystals. The excitation spectrum of the 3.82 eV band shows a new band within the 8–9 eV region. The band becomes most prominent for BaF2–3 wt% LaF3 and has a maximum at 8.7 eV at 290 K. The decay time for the 4.26 eV emission band of BaF2–3% LaF3 considerably increased with decreasing temperature like the exciton decay time for pure BaF2 ,w here the decay time of unperturbed excitons grows to 121 µ sa t 17 K. In contrast to this, the decay time of the 3.82 eV band increases from 4.1 µ sa t 290 K to 6.2 µ sa t1 5K .T he significant reduction of the triplet–singlet decay time can be associated with strong interaction of excited electrons with lanthanum ions. It was concluded that the 4.26 eV emission band is associated with weakly perturbed excitons, while the 3.82 eV band is associated with strongly perturbed excitons. The possible spatial configurations of excitons are discussed.

In this work, we carry out investigation of the suppression of the self-trapped exciton emission in BaF2:La crystals with various La concentrations (up to 5%), including measurements of thermo-luminescence, optical absorption of hole centers in the temperature range from 77 K to room temperature. Exciton luminescence was also investigated. Two luminescence bands at 4.0 and 3.67 eV were observed. It was concluded that the 4.0 eV emission band belongs to weakly perturbed excitons, while the 3.67 eV band belongs to strongly perturbed excitons.

Defects in Ce3+- and Eu2+-doped alkaline-earth fluorides, created by vacuum ultraviolet (UV) photons at energies lower than at the bandgap, were investigated by various methods: thermostimulated luminescence, photostimulated luminescence and optical absorption. The CaF2:Eu2+ thermoluminescence curves in the range of 60-330 K due to various types of trapped holes were the same after vacuum UV illumination as after x-irradiation. Thermoluminescence curves of Ce3+-doped alkaline-earth fluorides created by vacuum UV illumination or x-irradiation were generally similar. However, Vk thermoluminescence peaks were absent in vacuum UV illuminated CaF2:Ce3+ and SrF2:Ce3+ crystals. This fact is obviously associated with the presence of charge-compensating fluorine interstitials in Ce3+-doped crystals. The creation of Ce2+ characteristic bands was observed in photostimulated luminescence spectra as well as in optical absorption spectra of vacuum UV illuminated or x-irradiated Ce3+-doped crystals. The suppression of hole thermoluminescence peaks in CaF2:Eu2+ crystals by blue light is due to the photoionization of Eu+ ions. The proposed mechanism for the creation of trapped hole and trapped-electron defects by vacuum UV illumination involves charge-transfer-type transitions, in which the electron transfers from the valence band to an impurity level lying in the bandgap. Comparison of all energies involved of transitions in the crystals investigated shows that the sum of all the transition energies is less than that of the bandgap by 1.5-3.5 eV. This energy difference can be considered to be the energy of lattice relaxation around the created Ce2+ or Eu+ ions.