Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr → NaCl → KI → NaBr → CsBr → RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.     

Charge-Transfer Processes in Doped Alkali Halides

Defects formation under UV-irradiation in the impurity-induced absorption bands at 4.2 K has been studied for crystals with and without traps for electrons (CsI:Pb and Eu^2?-doped alkali halides, respectively). In both cases the results have been explained by an electron transfer from the impurity-perturbed halogen ion states, resulting in the appearance of electrons and holes in the crystal. In CsI:Pb, the electrons are trapped by lead ions and the holes are self-trapped. In Eu^2?-doped crystals, the electrons and the holes recombine with the formation of excitons, whose decay results in the creation of Frenkel defects.     

Enrichment centers in sodium bromide crystals

The primary products of irradiation of alkali halide crystals are anion and cation Frenkel pairs [1–4]. Interaction of these products among themselves and with structural defects existing before irradiation leads to the appearance of various structural defects, the majority of which are optically active. The present study will treat enrichment centers which can be formed from the components of Frenkel cation pairs. Studies of this type are important in providing information on states in which radiation-created cation defects can exist stably in the lattice.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 119–121, August, 1979.     

Laws governing the creation of electronic color centers in LiF crystals acted on by pulsed irradiation

The processes of creating and transforming electronic color centers in an LiF crystal irradiated with a nanosecond electron pulse are investigated using pulse spectrometry with nanosecond resolution for times in the range 10⁻⁸ to 10⁵ sec. It is shown that the thermally activated mechanism of forming Frenkel pairs in the 12–200 K range consists of successively creating exciton states, as the temperature rises, having different degrees of spatial separation of the electron and hole components. It is concluded that the structure of self-trapped excitons evolves as a function of temperature and time, and that this evolution commences for any alkali halide crystal with the creation of self-trapped excitons ofD _2h point symmetry at 4 K. It is established that the interaction of electronic excitations with electronic color centers changes the properties of both the electronic excitations themselves and the color centers. In a crystal containing neutral electronic centers there is a fall in the yield of self-trapped excitons and Frenkel pairs and an increase in the contribution of the radiative channel for loss of the irradiation energy by the color centers. A mechanism is proposed for exciting luminescence of electronic color centers. It is established that short-lived irradiation-induced states exist, in particular a change in the spin state or in just the energy state of a center in the irradiation field, and that the appearance of these states changes the efficiency and directivity of the charge evolution of the electronic color centers. State Architectural Building Academy, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 57–75, November, 1996.     

Transient optical absorption and luminescence in <Emphasis Type="Italic">A</Emphasis>Pb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> (<Emphasis Type="Italic">A</Emphasis> = K,Rb) crystals

This paper reports on a study of transient optical absorption and pulsed cathodoluminescence in APb₂Cl₅ (A = K, Rb) in the visible and ultraviolet spectral regions. The measurements performed by absorption optical spectroscopy with nanosecond time resolution showed the transient optical absorption of APb₂Cl₅ to derive from optical transitions in hole centers, and that the optical density relaxation kinetics is mediated by interdefect tunneling recombination in complementary pairs which involves Frenkel defects on the cation sublattice and self-trapped carriers. The slow components in the transient optical absorption decay kinetics, with characteristic times ranging from a few ms to seconds, have been assigned to diffusion-mediated annihilation of interstitial atoms with alkali metal vacancies. The mechanisms underlying creation and relaxation of the short-lived Frenkel defects on the cation sublattice and self-trapped carriers have been analyzed.     

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F ⁻ and Cl ₃ ⁻ -center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed. Fiz. Tverd. Tela (St. Petersburg) 41, 433–441 (March 1999)     

A comparative analysis of the spectral characteristics of triplet self-trapped excitons and F 2 centers in alkali halide crystals

A comparative analysis of the spectral characteristics of self-trapped excitons (STE) and F ₂ centers in the states with the same spin multiplicity is carried out. Based on the analysis, a criterion for the separation of the triplet-triplet (T-T) absorptive transitions in the electronic and hole components of the STE in any alkali halide crystal is proposed. It is concluded that inhomogeneities in the form of a homological cation or anion impurity in the nearest coordination shells of the spatial position of the STE, rather than hole, affect the spectral position of the T-T transitions in the electron component of the STE.     

Radiation effects in ionic solids

Current development of the research of radiation damage in ionic solids is reviewed. Emphasis is placed on the correlation between elementary radiation damage processes and the atomic and electronic structures of the materials. Both the radiation damage induced by electronic excitation and by elastic collision are treated. For the former two crucial processes, the self-trapping of excitons and the formation of stable Frenkel pairs from the self-trapped excitons in several materials, is discussed in terms of the structures of materials. Deficiency in the available data on the knock-on threshold energies are pointed out. Available information of Frenkel pairs produced by electronic and elastic encounters is surveyed. Possible models of defect clustering are summarized and existing information on clustering is discussed on their basis.     

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.     

Creation and optical properties of stable and metastable hole-trapped centres in beryllium oxide

Abstract

The paper is devoted to study of formation mechanisms and optical absorption of the hole-trapped centers in neutron, electron-impulse and X-irradiated BeO crystals. V⁰ and V^? centers are found out to be formed as a result of neutron irradiation creating cation Frenkel pairs. Within the transient absorption decay kinetics, we registered a component whose thermal-time properties coincide with those of the luminescence of triplet self-trapped excitons. A number of absorption bands from the V_B center and exciton hole nucleus are interpreted as transitions between the O^? ion p-levels splitted by the crystal field, as well as polaron transitions and transitions into the valence band.     

Formation of elementary radiation defects in halide crystals with different types of crystal lattice

Pulsed spectrometry with time resolution has been used to study processes for creation of self-localized excitons and F centers under the action of a pulsed electron flux of nanosecond duration at temperatures in the region from 80 to 600 K in CaF₂ and LiF crystals. It is shown that an alternative to creation of self-localized excitons in the triplet state with increased crystal temperature during irradiation is formation of an F-H pair. A comparative analysis of the effectiveness of the processes for creation of elementary defects in the crystals studied is given.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 13–19, January, 1995.     

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract

A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Temperature and impurity concentration dependences of the efficiency of Frenkel defect accumulation in alkali halide crystals

A simple phenomenological model of the efficiency of accumulation of F centers in alkali halide crystals as affected by temperature and homologous cation impurity is considered. The conclusion is drawn that the temperature dependence is probably due to elastic interaction of primary Frenkel defects — F , H centres affecting their diffusion-controlled separation (similar to metals) whereas the impurity concentration dependence is due to the tunneling recombination of F , H_A pairs.     

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.     

The efficiency of formation of primary radiation defects in LiF and MgF2 crystals

Processes of radiation formation of primary defects—F centers and self-trapped excitons—in lithium and magnesium fluorides, which have crystal lattices of different types and similar widths of the band gap and valence band, have been studied in a wide temperature range (11–500 K). It is shown that, along with qualitative similarity of the regularities of formation of the defects under study, LiF and MgF₂ crystals are characterized at low temperatures (11–100 K) by different relationships between the energy dissipation channels for self-trapping electronic excitations and the types of self-trapped excitons arising.     

Detection of defects binding free polarons in colored alkali halide crystals

The assumption has been made that defects binding free polarons in colored alkali halide crystals are F'-center, i.e., defects that slow down the motion of dislocations (photoplastic effect). This assumption has been confirmed by the experiments performed in this study. Thus, the anion vacancy in alkali halide crystals at a low temperature can capture three electrons: two electrons at a deep level (F'-center) and one electron in a bound polaron state. This electron is retained due to the energy gain in the interaction of a local deformation of the polaron and a local deformation surrounding the F'-center, despite the presence of the Coulomb repulsion.     

Quantum-chemical simulation of Frenkel pairs separation in a LiF crystal

Semiempirical quantum-chemical (INDO) simulation of the creation of the primary Frenkel pairs of defects in a LiF crystal based on the Pooley-Smoluchowski mechanism of the self-trapped exciton annihilation has been undertaken. The conclusion has been drawn that this mechanism can be operative from the viewpoints of both the time and the energy needed to create the F, H pair in terms of this mechanism. Unlike the Toyozawa's model an initial vibrational excitation is not bottleneck of the decay process (cf. [8]).     

Excitation of self-trapped-exciton luminescence in the recombination of frenkel defects in BeO

Polarized luminescence and transient optical absorption (TOA) induced by pulsed electron irradiation in beryllium oxide crystals were studied. Exponential stages with decay times τ = 6.5 ms were observed to exist in luminescence bands at 4.0, 5.0, and 6.7 eV, which coincide in spectral composition and polarization characteristics with the luminescence of self-trapped excitons (STEs) of two types. The formation efficiency of centers with a 6.5-ms decay time is comparable to that of triplet STEs. The general characteristics of the kinetics and the decay times of the TOA of these centers do not depend on electron fluence and are governed by the monomolecular recombination process. The spectra of TOA centers with a decay time of 6.5 ms were found to be similar to those of V-type hole centers and STE hole components. The mechanism by which recombination of closely spaced, spatially correlated Frenkel pairs, Be⁺ and V^? centers, brings about an exponential component with a 6.5-ms decay time in the luminescence of STEs of two types in BeO is discussed.