M. Kirm

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO 4 and CdWO 4 crystals, measured using synchrotron radiation (4-30 eV) at 8 K, have been analysed. In CaWO 4 the IL (2.9 eV) is efficiently excited by photons of 5.5-7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hv>6.8 eV). The energy threshold for the multiplication of electronic excitations is E t 15 eV in CaWO 4 and E t 10 eV CdWO 4 . In CdWO 4 , a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO 4 crystals in the region of 2-5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons.

Investigation of Cu-doped Li2B4O7 single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate (Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance (EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and 1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired hole with strong d-character and characteristic hyperfine splittings can be ascribed to Cu2+ substituted at a Li lattice site and displaced due to relaxation. The results on the Cu+-related luminescence strongly support the conclusion about a low-symmetry position of copper impurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics of the Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of the Cu+ centres. An off-centre position of the Cu+ ion in the relaxed excited state is suggested.

Exciton–Exciton Interaction in CdWO

Using time-resolved luminescence spectroscopy, excitation density effects were investigated under conditions of resonant creation of excitons by femtosecond laser pulses in CdWO4 scintillator at room temperature. It was revealed that the decay kinetics of intrinsic emission becomes accelerated and deviates from the exponential law in the initial stage due to the Förster dipole-dipole interaction of self-trapped excitons. It is shown that these processes originate from the relaxed exciton states even at excitation power up to ~1012 W/cm2. The quantitative analysis of the experimental data is performed, and the dipole-dipole interaction parameters discussed. The results obtained are shown to be of general importance for understanding the problem of nonproportional response of scintillators, whose operation is based on intrinsic luminescence.

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Abstract The excitation spectra of luminescence of Ag + and Tl + centres as well as of self-trapped excitons have been measured in a spectral region from 5 to 30 eV for RbCl:Ag and RbCl:Tl crystals at 8 K. The comparative analysis of these spectra and photoelectric characteristics of RbCl thin films confirms that the effect of impurity centre excitation by hot conduction electrons exists in wide-gap ionic crystals. Hot photoelectrons excite Ag + and Tl + impurity ions and also create near-impurity electronic excitations. Possible application of fast processes with the participation of hot photoelectrons and hot photoholes in luminescent detectors of radiation are discussed.

Under Resonant Excitation by Intense Femtosecond Laser Pulses

The firing voltage (FV) of gas discharge in a test cell of plasma display materials was investigated for standard protective layers of MgO deposited by electron beam, and ternary BaY2O4 and BaGa2O4 oxides grown by pulsed laser deposition on the special dielectric coated glass substrates. The determined FVs for MgO (160 V), BaY2O4 (210 V) and BaGa2O4 (257 V) lead to the conclusion that a replacement of MgO by these ternary oxides is not expedient for plasma display panels. Using results from luminescence spectroscopy, values for the energy gap Eg ≈ 6.2 and 5.8 eV were estimated for BaY2O4 and BaGa2O4, respectively. The main reason for the observed high FVs is attributed to strong electron affinities χ, where χ (BaY2O4) < χ (BaGa2O4).

Direct excitation of impurity centres by hot photoelectrons in ionic crystals

The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for α-Al2O3, Al2O3:Sc and Al2O3:Ga crystals have been measured by means of synchrotron radiation of 4–40 eV at 8 and 80 K. The luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (τ<2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25–37 eV leads to the formation of two or three electron-hole pairs causing a sharp increase of the efficiency of extrinsic emission of Sc3+ (5.6 eV), Ga3+ (4.6 eV) and F+ centers (3.85 eV).

Investigation of possible replacement of protective magnesium oxide layer in plasma display panels by barium ternary oxides

Using free electron laser excitation in the XUV range, CaWO4 samples were exposed to ultrashort intense photon pulses (photon energy, 89.84 eV; average pulse energy, 10 μJ; pulse length, 25 fs), and their luminescence was studied with time-resolved spectroscopy. In the decay curves measured in the temperature range 8–300 K, a nonexponential emission decay with shortening of the lifetimes over the first few microseconds was observed, depending on the excitation density. Using a model for dipole-dipole interaction of excitons under nonuniform excitation densities, the structure of the decay curves can be reproduced in good agreement with the experimental data, and parameters for the initial exciton interaction can be calculated.

SPECTRAL-KINETIC STUDY OF SELF-TRAPPING AND MULTIPLICATION OF ELECTRONIC EXCITATIONS IN AL2O3 CRYSTALS

Homologue scheelite crystals CaWO4, SrWO4, and BaWO4 possess similar crystal and electronic structure, but their luminescence exhibits drastically different thermal stabilities. By measuring the temperature dependence of the decay time of the intrinsic luminescence and fitting it to a three level model, we have qualitatively shown the effective exciton radius to increase in the order CaWO4 → SrWO4 → BaWO4, which explains the differences in the thermal stability. The origin of the variation in the exciton radii is suggested to be related to differences in the excited state dynamics in these crystals. From the decay kinetics measured under conditions of high excitation density, the efficiency of dipole-dipole interaction between excitons is shown to grow with exciton delocalization.

Self-quenching effects of excitons in CaWO4 under high density XUV free electron laser excitation

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.

Cation influence on exciton localization in homologue scheelites

The first observation and characterization of Lu3+ 4f135d-4f14 luminescence from the CaF2: Lu3+ crystal are reported, and the multisite structure in the spectra of Ce3+, Gd3+, and Lu3+ ions in the CaF2 host is analyzed with the high-resolution VUV spectroscopy technique using synchrotron radiation. It is shown that vibronic structure in the emission and excitation spectra of interconfigurational transitions in Gd3+ and Lu3+ ions doped into CaF2 differs from that observed for Ce3+ ions entering mainly at the tetragonal (C4v) sites. However, the exact types of sites in which the Gd3+ and Lu3+ ions reside in a CaF2 lattice cannot be identified using only the obtained experimental spectroscopid data.