Comparative analysis of the self-localization properties of excitons and F2 centers in alkali halides

A comparative analysis is presented of the energetic and temporal parameters of self-localized excitons and F₂ centers in the triplet state in a number of crystals. We conclude that in crystals with different values of the Rabin-Klick parameters and self-localized excitors exhibit D_2h symmetry in the triplet state.Translated from Izvestiya Vysshikh Uchenbykh Zavedenii, Fizika, No. 8, pp. 115–118, August, 1995.     

Recombination luminescence of KI crystals with a sodium impurity

It is shown in this paper that both electron-hole and hole -electron recombination luminescence are excited effectively in KI Na crystals at low temperatures. In addition to the radiation of self-localized excitons and of excitons localized at the sodium ions, the luminescence of excitons localized at dual sodium centers is detected.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 63–66, February, 1975.     

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.     

Relaxation dynamics of electronic excitations in CaWO4 and CdWO4 crystals studied by femtosecond interferometry technique

The relaxation of electronic excitations in CdWO₄ and CaWO₄ crystals was studied using the method of time-resolved interferometry with 100-fs temporal resolution at temperatures 15–295 K. The electronic system was excited in the one-photon and two-photon regime within the excitonic band in CaWO₄ and in the electron-hole continuum in CdWO₄. Immediate trapping of charge carriers was detected under pumping in the excitonic band of CaWO₄. This result is in agreement with decay kinetics measurements with nanosecond time resolution under direct creation of excitons by 100-fs laser pulses. Fast relaxation of charge carriers followed by formation of excitons was observed in CdWO₄. The comparison with previous work allows suggesting the formation of bulk excitons and surface-perturbed excitons in the multi-photon and one-photon regime. The corresponding models of self-trapped exciton creation in tungstate crystals are discussed.     

Temperature variation of the optical absorption edge for ammonium aluminum alum

We have studied the absorption spectra of NH₄Al(SO₄)₂·12H₂O crystal hydrates in the visible region of the spectrum, near the low-temperature phase transition at T_c = 76 K. We have shown that the absorption edge in these materials has an exponential shape, following the empirical Urbach’s rule as a result of exciton-phonon interaction. The relatively strong exciton-phonon interaction is associated with a band formed by self-localized (self-trapped) excitons. The experimental data obtained support the presence of a phase transition in the studied ammonium alum at 76 K. We have shown that ammonium aluminum sulfate dodecahydrate crystals are promising materials for fabricating laser frequency converters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 258–262, March–April, 2007.     

Creating primary defects in calcium fluoride crystals with various prehistories, using pulsed electron irradiation

Optical spectroscopy with nanosecond resolution has been used to study how the prehistory (the presence of impurities, heat treatment) of CaF₂ crystals affects the processes of creating autolocalized excitons (AEs) under the influence of a pulse of accelerated electrons. The breakdown of the dispersivity condition for the creation of AEs is detected in a nominally pure, nonheat-treated crystal. It is shown that the initial structural defects play a defining role in creating the initial defect density. A model of the radiation disordering of an actual fluorite crystal is constructed. An analogy is traced to the processes of impurity and thermal disordering. Fiz. Tverd. Tela (St. Petersburg) 41, 442–450 (March 1999)     

Non-radiative decay of excitons on ionized donors and acceptors in semi-conducting and dielectric crystals

The possible processes of non-radiative decay of excitons with a large radius on ionized donors and acceptors in semi-conducting and dielectric crystals are theoretically investigated in the first approximation by means of time perturbation calculus of quantum mechanics using the method of second quantization. Theoretical relations are found for the probability per sec of non-radiative decay of excitons on ionized acceptors in Cu₂O crystals and on electron trapping levels in CdS crystals.     

Trapping and self-trapping in ytterbium-doped oxides with charge transfer luminescence

Temperature dependence of the charge transfer luminescence (CTL) of Yb-doped yttrium aluminum garnet Y₃Al₅O₁₂-Yb (YAG-Yb) and Yb-doped lutetium aluminum perovskite LuAlO₃-Yb (LuAP-Yb) crystals under X-ray excitation and their thermostimulated luminescence are investigated in the temperature range 30-350 K and compared to those of undoped crystals. Simulation using a set of kinetic equations describing the processes of creation of excitons, electron-hole pairs, their trapping and self-trapping, radiative relaxation and quenching is presented for the systems under investigation to analyze qualitatively two different types of experimentally observed temperature dependences: CTL yield decline with the temperature decrease below 110 K as in case of YAG-Yb and constant yield in the same temperature range as in case of LuAP-Yb.     

Self-trapped excitons in low-symmetry fluoride crystals

Triplet self-trapped excitons have been detected in low-symmetry crystals of the fluorides BaFCl₄, BaFBr, and LaF₃. In BaFBr crystals two types of excitons, on-center and off-center (with a symmetric or displaced configuration). The slow initiation of exciton luminescence in BaFBr is associated with the barrier transition from an on-center to an off-center configuration. In LaF₃ crystals tunnelling population of the exciton state also appears in the initiation. In LaF₃ crystals excitons appear only in the luminescence and excitation spectra while in the absorption spectra the exciton transitions are concealed beneath band-to-band transitions. A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–56, November, 1996.     

Recombination-assisted creation of cation excitons and cross-luminescence quenching in CsCl crystals at high excitation densities

At high excitation densities, recombination-assisted creation of cation excitons, which transfer energy efficiently to the anion sublattice to initiate the luminescence of anion excitons and impurity centers, has been observed in CsCl crystals. At the same time, the creation of cation excitons competes with the electron recombination with cation holes and quenches the cross-luminescence. The intensity ratio of the cross-luminescence to exciton-impurity luminescence is different for crystal irradiation with γ rays and heavy particles.     

Self-trapped excitons in LiB3O5 and Li2B4O7 lithium borates: Time-resolved low-temperature luminescence VUV spectroscopy

Results are reported of a coordinated investigation of the dynamics of electronic excitations in LiB₃O₅ and Li₂B₄O₇ crystals by low-temperature luminescence VUV spectroscopy performed with subnano-second resolution under synchrotron photoexcitation. Data on the photoluminescence (PL) decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra obtained at 295 and 9.6 K are reported for the first time; the PL of the borates in the 3.5-eV region caused by radiative annihilation of self-trapped excitons (STE) has been established to have an intrinsic nature; the σ and π STE luminescence bands originating from singlet and triplet radiative transitions have been isolated; the shift of the STE σ band relative to the π band has been interpreted; the LBO recombination luminescence band has been isolated; and the creation and decay channels of relaxed and unrelaxed excitons in lithium borates are discussed.     

Exciton luminescence of 8-azasteroid microcrystals

Luminescence of microcrystals of 2,3-methoxy-8-azagon-1,3,5(10),13-tetraene-12,17-dion of the class of molecules of biologically active steroids is detected at room temperature (293 K). It represents fast fluorescence of free and self-localized excitons and prolonged phosphorescence of triplet excitons. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 440–443, May–June, 1999.     

Radiative relaxation of low-energy electron excitations and point defects in beryllium oxide

The mechanism by which elementary point defects arise and evolve in beryllium oxide crystals and the latest advances in the dynamics of electron excitations of this compound are considered. Experimental and theoretical results which touch on the generation and evolution of elementary point defects are obtained. The properties of intrinsic and impurity point defects, and the crystal-chemical fundamentals of defect generation, optical and luminescent transitions in them, and color centers are discussed. The electron structure of B²⁺, Al²⁺, Zn⁺, and [Li]⁰ defects, nonisothermal relaxation of paramagnetic centers, thermostimulated recombination processes, and the concept of the coexistence of two types of self-localized excitons in beryllium oxide, radiative annihilation of which gives rise to luminescence in the UV and far UV bands are considered. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–93, November, 1996.     

Exciton-phonon interaction in Y2O3 and Sc2O3 thin films

An investigation is made of the fundamental absorption edge of Y₂O₃ thin films. Based on its temperature dependence the exciton-phonon interaction is studied, which makes it possible to interpret the absorption edge as the absorption of self-localized excitons. A number of parameters of the energy spectrum of the investigated films are determined. I. Franko State University, 50, Dragomanov St., L'vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 132–134, January–February, 1999.     

Time-resolved spectroscopy of self-trapped excitons in fluorides of alkaline-earth metals under pulsed electron irradiation

Nanosecond-resolution absorption spectroscopy at room temperature was used to study the laws governing the creation and evolution of the primary defect structure in CaF₂, SrF₂, and BaF₂ crystals exposed to an accelerated electron pulse. It is shown that the spectral-kinetic characteristics of self-trapped excitons created in undamaged parts of the crystal lattice are qualitatively similar. Partial polarization of the absorption of self-trapped excitons is observed in CaF₂. The structure of the transient absorption spectra becomes more complex in the sequence CaF₂, SrF₂, BaF₂ because of the formation of excitons trapped in phase inclusions of homologous cationic impurities. The spectral characteristics of excitons trapped in undamaged parts of the CaF₂ and SrF₂ lattice and in their phase inclusions in BaF₂ are the same although the latter have a considerably shorter relaxation time. Short-lived (τ⩽100 ns) absorption of unknown defects was observed in the spectral range ⩾5 eV. Fiz. Tverd. Tela (St. Petersburg) 40, 1228–1234 (July 1998)     

Luminescence centers in Sc2O3

Sc₂O₃ luminescence spectra are studied. The spectra are separated into elementary bands by the Alentsev–Fock method. It is established that the luminescence spectra consist of a number of overlapping bands with maxima at 3.5; 3.05; 2.65; 2.35, and 2.05 eV. The band at 3.5 eV is interpreted as emission of self-localized excitons, and the other bands, as defect-center recombination. L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 776–778, November–December, 1997.     

Emission spectra for ArNe binary crystals

Emission spectra for ArNe binary crystals have been measured in the range of 6–18 eV at the Ar concentration of 0.01 – 100 mol%. Luminescence was excited by slow electrons at T = 5°K. The spectrum of excitons bound on the atomic and molecular type centres is observed. The energies of radiative transition in a crystal appear to be very close to those in a gas phase because of the relaxation processes at which the resonance exchange interaction decreases considerably. The bands of atomic centres display a fine structure which reflects the effect of the lattice crystal field of Ne. For the ArNe crystals it has been shown that excitons are not thermalized completely and there is the independent relaxation of the lowest states of various Wanier series to local excitons. A scheme for the relaxation process in argon involving the formation of a molecular centre Ar₂¹ is given.     

Vacuum ultraviolet and X-ray emission spectroscopy of anion and cation excitons in oxide crystals

The results of a combined experimental investigation into the radiative relaxation of anion and cation excitons during selective vacuum ultraviolet-(up edge) and soft X-ray (core) excitation of berylliumcontaining BeO, Be₂SiO₄ and La₂Be₂O₅ crystals are presented. They indicate that the relaxation of anion and cation excitons in multi-component oxides occurs in the same local fragments of the crystal lattice, due to their higher predisposition (in comparison with other fragments) to short-lived distortions. It has been found that in La₂Be₂O₅ crystals characterized by a considerable difference in the ionic radii of cations (beryllium and lanthanum), the beryllium-oxygen tetrahedra do not participate in exciton relaxation processes.     

Luminescence centers in thin films of lead and bismuth tungstates

Luminescence spectra of thin films of PbWO₄ and Bi₂WO₆ are invesigated. The Alentsev-Fock method is used to separate the spectra into elementary components. The emission bands with maxima at 2.8 eV in PbWO₄ and at 2.93 eV in Bi₂WO₆ luminescence spectra are interpreted as the emission of self-localized Frenkel excitons. The bands with maxima at 2.35 and 1.75 eV in PbWO₄ and at 2.35 and 1.9 eV in Bi₂WO₆ are related to oxygen vacancies. L’vov State University, 50, Dragomanov St., L'vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 143–145, January–February, 1998.     

Low-temperature photoluminescence in AgGaSe<Subscript>2</Subscript> single crystals

Photoluminescence spectra from a single-crystalline AgGaSe₂ ternary compound grown by the Bridgman-Stockbarger method from a nonstoichiometric melt are studied in the temperature interval 8–300 K under various excitation levels. The spectra contain emission bands associated with donor-acceptor recombination, as well as with bound and free excitons. The exciton binding energy and the energy gap of the AgGaSe₂ crystals are evaluated. The temperature dependence of the energies of bound and free excitons, as well as of the energy gap of the crystals, is constructed.