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.     

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.     

Metastable optical absorption of relaxed electronic excitations in BeO-Zn crystals

Spectra of metastable optical absorption and its relaxation kinetics have been studied in zinc-doped BeO crystals by time-resolved pulsed absorption spectroscopy. A comparison of the observed induced optical absorption of self-trapped excitons and small-radius excitons bound to the zinc impurity suggests that their hole components have similar structures and reveals distinctive features of “forbidden” optical transitions in the electronic components. Metastable optical absorption in Zn⁺ centers has been discovered. It is shown that the small-radius excitons bound to the zinc impurity form in the hole stages of thermally stimulated tunneling recombination processes involving Zn⁺ electronic centers. It has been found that the high recombination probability of the electronic and hole centers created in BeO-Zn crystals by an electron beam may be due to the high degree of their spatial correlation. Fiz. Tverd. Tela (St. Petersburg) 41, 601–605 (April 1999)     

Energy transfer in Gd2SiO5-Ce, Y2SiO5-Ce, and Be2La2O5-Ce crystals during selective VUV and core excitation

Luminescence vacuum ultraviolet time-resolved spectroscopy is used to study electronic excitations and energy transfer in Ce³⁺-doped crystals of gadolinium and yttrium oxyorthosilicates excited by synchrotron radiation in the vacuum ultraviolet (4–30 eV) and x-ray (50–200 eV) regions. At T = 10 K, both crystals exhibit intrinsic electronic excitations whose radiative relaxation occurs through fast (τ = 3 ns) and slow (microsecond) channels, which correspond to two possible types of self-trapped excitons. A comparison of the relaxation of above-edge and core electronic excitations in the Ce³⁺-doped crystals of gadolinium oxyorthosilicate and lanthanum beryllate indicates that the nature of the charge carriers involved in the recombination processes of energy transfer to luminescence centers is diverse. __________ Translated from Fizika Tverdogo Tela, Vol. 47, No. 8, 2005, pp. 1435–1439. Original Russian Text Copyright ? 2005 by Ivanov, Pustovarov, Kirm, Shlygin, Shirinskii.     

Hole-electron mechanism of F-H pair generation in RbCl crystals with impurity electron traps

Production of F, Cl ₃ ⁻ , Ag⁰, and Tl⁰ centers in RbCl:Ag and RbCl:Tl crystals by photons having energies ranging from 5 to 10 eV has been studied at 295 and 180 K. It is shown that creation of near-impurity excitations is accompanied by formation of F centers localized in the vicinity of Ag⁺ and Tl⁺ ions. F centers are produced in direct optical generation of self-trapped excitons. In addition to the well-known mechanism of F-H pair production in nonradiative recombination of electrons with self-trapped holes, a hole-electron process has been revealed for the first time to operate in RbCl:Ag having deep electron traps. By this mechanism, F-H pairs appear in the following sequence of stages: thermally stimulated unfreezing of hopping diffusion of self-trapped holes (V _K centers), tunneling electron transfer from Ag⁰ to the approaching V _K centers, and subsequent nonradiative decay of triplet self-trapped excitons near Ag⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1238–1245 (July 1998)     

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.     

Color centers in heavily irradiated CsI(Tl) crystals

The absorption and luminescence properties of CsI(Tl) crystals colored by irradiation are studied by the method of the time-resolved spectroscopy. The scheme of the electron transitions in CsI(Tl) crystal is suggested to explain the appearance of the color centers under exposure to the near-UV light. It is established that either of the two types activator color centers holds the charge carrier with opposite sign. The model of the hole Tl²⁺v_c⁻ activator color center is suggested. According to the model the positive charge of Tl²⁺ ion is compensated by the negative charge of a close cation vacancy v_c⁻. The color center emission reveals in the cathode-luminescence spectrum of the colored CsI(Tl) crystal. The high-dose irradiation of CsI(Tl) crystal results in the reduction of the decay time of the near-thallium self-trapped excitons (STE) emission. The decay kinetics of Tl²⁺v_c⁻ emission contains the time components typical for the decay kinetics of near-thallium STE emission. The reason of the observed effects is the energy transfer from the near-thallium STE excitons to the color centers via the inductive-resonant mechanism.     

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.     

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.     

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.     

Short-lived primary radiation defects in LiF crystals

The spectral and kinetic parameters of electron-pulse-initiated transient absorption and emission of LiF crystals were studied using pulsed spectrometry with a nanosecond time resolution. The measurements were performed in the spectral region of 6 eV, the temperature range of 11–150 K, and within 10^?8–10 s after the termination of an electron pulse. It is shown that the electron-pulse irradiation not only gives rise to F, V _k , and H centers in the LiF crystal but also to certain short-lived defects of two types that differ in the spectral positions of the absorptive and radiative transitions, the lifetime, and the temperature dependence of the production efficiency. Defects of type I feature absorptive transitions at 5.5 and 5.1 eV and a radiative transition at 5.8 eV, whereas the absorptive transitions at 5.3 and 4.75 eV and a radiative transition at 4.4 eV are characteristic of type-II defects. It is found that a variation in the ratio between the concentrations of the different types of short-lived centers in the range of 11–150 K does not affect the quantum efficiency of the F centers. It is assumed that the observed centers are self-trapped excitons of various types.     

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.     

Localized electronic excitations in crystalline phenacite Be2SiO4

The results of coordinated spectroscopic studies of the nature and properties of electronic excitations localized at regular and defect sites of the Be₂SiO₄ lattice are presented. The methods employed are electron-beam-excited pulsed absorption spectroscopy, pulsed cathodoluminescence, and low-temperature VUV spectroscopy with selective excitation by synchrotron radiation. The bands in luminescence spectra of Be₂SiO₄ at 2.70 and 3.15 eV are assigned to [AlO₄]^5? and [SiO₄]^4? centers formed both in direct relaxation of electronic excitations at defect levels and through the formation of exciton-defect complexes. Disruptions of beryllium-oxygen bonds (short-lived defects in the form of beryllium vacancies V _Be ^? ) are considered as initiating the formation of optically active centers with characteristic absorption bands in the range 1.5–4.0 eV. The intrinsic luminescence of the Be₂SiO₄ crystal at 3.6 and 4.1 eV is attributed to radiative decay of self-trapped excitons of two types. A mechanism of exciton self-trapping at the [SiO₄] and [BeO₄] tetrahedral groups is proposed, which involves excitation transfer from a threefold-coordinated oxygen atom to neighboring silicon or beryllium atoms.     

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

It has been shown using atomic-force microscopy that the PbI₂ impurity is embedded in the CdI₂ crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the ³ P ₂ state of a free Pb²⁺ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI₂ crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI₂ in the CdI₂ crystal lattice.     

Electronic excitations and luminescence in CsLiB6O10 crystals

The paper presents the results of a complex investigation into the dynamics of electronic excitations in the CsLiB₆O₁₀ crystal (CLBO) by low-temperature luminescence VUV spectroscopy with subnanosecond time resolution under photoexcitation by synchrotron radiation. Strong broad-band low-temperature photoluminescence (PL) of the CLBO crystal has been revealed. Data on the PL decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra at 9.3 and 295 K are obtained. It is shown that the intrinsic PL of CsLiB₆O₁₀ in the 3.5-eV range is caused by radiative annihilation of self-trapped excitons. The channels of creation and decay of relaxed and unrelaxed excitons in cesium lithium borate are discussed. The band gap of CLBO is estimated as E _g≈8.5 eV. A monotonic increase in the excitation efficiency of intrinsic CLBO luminescence at exciting photon energies above 19 eV is identified as the photon multiplication process.     

Optical properties of UV-induced color centers in a KY3F10:Ce3+ crystal

The evolution of color centers induced in a KY₃F₁₀:Ce³⁺ crystal by UV radiation has been observed and interpreted. It has been revealed that, initially, the UV irradiation of the KY₃F₁₀:Ce³⁺ crystal induces the formation of color centers predominantly of the F-type, which, in a short time period of about ten minutes, are transformed into complex color centers of the F ₂-type, as well as into impurity color centers. Based on the data obtained, a diagram of energy states of the crystal, dopant, and color centers has been constructed, on which most probable processes that are caused by electronic transitions occurring in the KYF:Ce³⁺ crystal after its UV irradiation have been indicated.     

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.     

Short-living absorption and emission of CsI(Na)

This paper investigates the short-living absorption and the emission of CsI(Na) under a pulsed electron beam (Е_e=0.25 MeV, t_1/2=15 ns and W=0.003…0.16 J/cm²). The bands of singlet self-trapped excitons, as well as Na⁰ and V_k color centers have been detected in the transient absorption spectrum of CsI(Na). It has been found that the activator luminescence spectrum, peaking at 3.0 eV, fits a Gaussian (E_m=3.0 eV and FWHM=0.44±0.02 eV at 80 K) and remains the same at different time delays within 10⁻⁸-10⁻³ s. The decay kinetics of the 3.0 eV emission has one nanosecond exponential component and two microsecond ones with time constants 1.0 and 3.0 μs, which remain unchanged within 78-150 K. It is concluded that the activator emission is due to the radiative annihilation of sodium-perturbed two halide excitons from the non-relaxed singlet state. The pathways of such excitons creation are discussed.     

Collective excitations in a system of excitons

A high density system of Frenkel excitons is investigated under the condition that no type of condensation occurs in the system. The dynamic interaction between excitons and the excitonphonon interaction are considered. It is shown that sound-like collective excitations can exist in systems of excitons and excitons and phonons. The damping of excitations caused by finite life time of excitons is also taken into account.The author wishes to express his thanks to Prof. M.Trlifaj for suggesting the present theme and for useful discussions.     

Evolution of Frenkel pairs from excited states of self-trapped excitons in alkali halides

We find that the transition to the first and second electron excited states from the lowest self-trapped excitons in KCl and RbCl are π- and σ-polarized, respectively. For KCl the orientation of the H centers evolved from the self-trapped exciton by exciting to these states are shown to be in parallel to the orientation of the self-trapped excitons.