I.A. Kamenskikh

Optical and luminescent properties of anisotropic tungstate crystals

Features of the reflectivity spectra in the fundamental absorption region were analysed for a series of tungstates. The contribution of electronic states of cations to the formation of the bottom of the conduction band and the top of the valence band is demonstrated. Its relation with the dominant mechanism of the energy transfer to the emission centres and the nature of these centres is discussed.

Electronic structure and luminescence mechanisms in ZnMoO4 crystals.

Calculations of the band structure, partial densities of states and optical spectra of permittivity, reflectivity and absorption of perfect ZnMoO(4) crystal were performed using the full-potential linear-augmented-plane-wave method. It is shown that the calculated reflectivity spectra reproduce the main features of corresponding experimental spectra in the fundamental absorption region. The bandgap value of ZnMoO(4) is estimated as E(g) = 4.3 eV. Peculiarities of luminescence excitation spectra corrected for near-surface losses and losses on reflectivity are discussed, taking into account the results of the calculations. It is found that the energy structure of the lower part of conduction band is manifested in the excitation spectra of the intrinsic luminescence. The excitation spectra in the region 4.3-8.0 eV are formed by band-to-band electronic transitions mainly within the molybdate groups MoO(4)(2-), whereas electronic states of Zn(2+) cations are not directly involved into the excitation processes. It is shown that the structure of the intrinsic luminescence excitation spectrum depends on the temperature and mechanisms of the structure modification are discussed.

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Fast fluorescence and scintillation properties of cerium and praseodymium doped lutetium orthoborates

Abstract Both calcite and vaterite polyerystalline phases of LuBO3 doped with cerium and praseodymium have been prepared and their fluorescence properties investigated in the visible, near UV and VUV regions. The systems exhibit a strong and rapid near UV-blue fluorescence typical of the 5d → 4f parity-allowed transitions of dopants. It is shown that the excitation transfers from excitions and electron-hole pairs to dopants are very efficient. The light yield can be three times more efficient than that of the well-known Bi4Ge3O12 scintillator for the best samples.

Temperature dependence of the charge transfer and f?f luminescence of Yb3+ in garnets and YAP

For single crystals of YAG–15 mol% Yb, LuAG–15 mol% Yb and YAP–8 mol% Yb the temperature dependence of the charge-transfer (CT) and f–f luminescence of Yb3+ excited in the CT absorption band in the temperature range 7–300 K has been studied. Simulation of the experimental data taking into consideration three processes, photoionization of the CT state and radiative and non-radiative energy transfer to the 4f levels, has demonstrated that for adequate description of the relaxation of the CT state it is essential to get information on the temperature dependence of the direct excitation efficiency of the f–f luminescence as well as to study the role of intrinsic luminescence in the energy transfer to ytterbium ions. Excitation spectra of the CT and IR luminescence in the energy region 4–20 eV at RT and 10 K are presented and discussed.

Luminescence quenching as a probe for the local density of electronic excitations in insulators

Abstract Luminineence quenching which originates from the interaction of closely spaced electronic excitations (EEs: electrons, core- and valence-band holes, excitons, etc.) is considered. ‘Reactions’ between them at a distance shorter than that of dipole-dipole interaction are discussed. Creation of regions with high local density of EEs by VUV and XUV photons is demonstrated. Features of this type of quenching specific for different regions of excitation and different crystals are illustrated by experimental examples.

Anisotropy of optical properties of scheelite tungstates in the fundamental absorption region

Reflectivity spectra ofsingle crystals ofCa, Ba and Pb tungstates in the energy range 4–30 eV are presented. Anisotropy ofscheelite structure is studied on this set ofcrystals f or their different orientations using polarised

Crossluminescence in ionic crystals

Abstract Crossluminescence (CL) of ionic crystals is regarded with the account for the many-body effects. The crossluminescence yield in the pre-threshold region is considered on the basis of Fano interaction. Relaxation of the doubly-ionised 5p state of the cation ion as manifested in the CL and STE excitation spectra in the region of 4d core-hole creation is cliscussed. The effects of CL quenching are briefly reviewed.

High-energy excitation of luminescence of crystals with oxyanions

Abstract This article is a survey of the activity of the synchrotron radiation laboratory, Moscow University, in the application of synchrotron radiation to the investigation of luminescence excitation of insulators: Sn dioxide, Ca, Sr and Ba sulphates, zirconates, titanates and cerates. General tendencies of the luminescence quantum yield behaviour are considered. Computer simulation of the quantum yield is described.

CROSS LUMINESCENCE OF SEVERAL PEROVSKITE-TYPE CRYSTALS

Cross‐luminescence excitation and decay of crystals RbCaF3, CsCaCl3, and CsSrCl3 have been studied in the vacuum ultraviolet region. It has been shown that decay curves are not single exponential. The effect of structural phase transition on the cross‐luminescence decay time of RbCaF3 has been discovered.