Transient hole-polaron optical absorption in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Kinetics of the transient optical absorption in Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> and LiB<Subscript>3</Subscript>O<Subscript>5</Subscript> lithium borate crystals

This paper reports on a study of the kinetics of electron tunneling transport between electron and hole centers in Li₂B₄O₇ and LiB₃O₅ lithium borate crystals under the conditions where the mobility of one of the partners in the recombination process is thermally stimulated. A mathematical model has been proposed to describe all specific features in the relaxation kinetics of the induced optical density observed in Li₂B₄O₇ (LTB) and LiB₃O₅ (LBO) nonlinear optical crystals within a broad time interval of 10⁻⁸−1 s after a radiation pulse. The results of calculations have been compared with experimental data on transient optical absorption (TOA) of LTB and LBO crystals in the visible and ultraviolet spectral regions. The nature of the radiation defects responsible for TOA and the dependence of the TOA decay kinetics on temperature, excitation power, and other experimental conditions have been discussed.     

Transient optical absorption and luminescence in Li2B4O7 lithium tetraborate

This paper reports on a study of the transient optical absorption exhibited by Li₂B₄O₇ (LTB) in the visible and UV spectral regions. Using absorption optical spectroscopy with nanosecond time resolution, it is established that the transient optical absorption (TOA) in these crystals originates from optical transitions in hole centers and that the kinetics of the optical-density relaxation is controlled by interdefect tunneling recombination, which involves these hole centers and electronic Li⁰ centers representing neutral lithium atoms. At 290 K, the Li⁰ centers migrate in a thermally stimulated, one-dimensional manner, without carrier ejection into the conduction or valence band. The kinetics of the pulsed LTB cathodoluminescence is shown to be controlled by a relaxation process connected with tunneling electron transfer from a deep center to a small hole polaron migrating nearby, a process followed by the formation of a self-trapped exciton (STE) in an excited state. Radiative annihilation of the STE accounts for the characteristic σ-polarized LTB luminescence at 3.6 eV, whose kinetics is rate-limited by the tunneling electron transfer.     

Recombination kinetics in nonlinear defective LiB3O5 crystals

A study of recombination kinetics in LiB₃O₅ (LBO) crystals by time-resolved luminescence and absorption spectroscopy is reported. An investigation of the kinetics of transient optical absorption (TOA) and luminescence under ns-scale electron-beam excitation performed within a broad temperature range of 77–500 K and a 1.2–5-eV spectral interval has established that the specific features in the recombination kinetics observed in LBO involve electronic, B²⁺, and hole, O⁻, trapping centers. The TOA and luminescence kinetics, as well as their temperature dependence, are interpreted by a model of competing hole centers. Relations connecting the kinetics parameters and the temperature dependence to the parameters of the main LBO point defects are presented. Fiz. Tverd. Tela (St. Petersburg) 40, 2008–2014 (November 1998)     

Short-living defects and recombination processes in Li6Gd(BO3)3 crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium-gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and ultraviolet spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermally stimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow TOA decay kinetics components, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-living Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Kinetics of electron tunneling transfer in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) on a wide temporal scale (10⁻⁸–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.     

Nonradiative relaxation of photoexcited O<Stack><Subscript>1</Subscript><Superscript>0</Superscript></Stack> centers in glassy SiO<Subscript>2</Subscript>

The processes involved in the excited-state relaxation of hole O ₁ ⁰ centers at nonbridging oxygen atoms in glassy SiO₂ were studied using luminescence, optical absorption, and photoelectron emission spectroscopy. An additional nonradiative relaxation channel, in addition to the intracenter quenching of the 1.9-eV luminescence band, was established to become operative at temperatures above 370 K. This effect manifests itself in experiments as a negative deviation of the temperature-dependent luminescence intensity from the well-known Mott law and is identified as thermally activated external quenching with an energy barrier of 0.46 eV. Nonradiative transitions initiate, within the external quenching temperature interval, the migration of excitation energy, followed by the creation of free electrons. In the final stages, this relaxation process becomes manifest in the form of spectral sensitization of electron photoemission, which is excited in the hole O ₁ ⁰ -center absorption band.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

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.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Spectral properties of a Nd<Superscript>3+</Superscript>-doped Li<Subscript>3</Subscript>Ba<Subscript>2</Subscript>Gd<Subscript>3</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>8</Subscript> crystal

The spectral properties of a promising laser material, ternary molybdate Li₃Ba₂Gd₃(MoO₄)₈:Nd³⁺, are studied (i.e., its optical absorption spectra, luminescence spectra, kinetic of luminescence decay, and temperature dependence of luminescence). Luminescence of the crystalline matrix is detected, and the temperature dependence of its intensity and reabsorption by neodymium are investigated.     

Transient optical absorption induced by an electron pulse in KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystals

The efficiency of formation and time evolution of radiation-induced structural defects and pulsed luminescence in KPb₂Cl₅ crystals under the action of a single electron pulse (E = 250 keV, τ = 20 ns) have been investigated. The spectra (1.1–3.8 eV) and relaxation kinetics (time interval 5 × 10^?8?5 s) of transient optical absorption and the pulsed cathodoluminescence spectra and decay kinetics (1.4–3.1 eV) have been measured in the temperature range 80–300 K. It is revealed that the induced optical density and its time evolution depend strongly on temperature, and the absorption relaxation time contains several components and reaches several seconds at T = 300 K. The decay kinetics of transient absorption and pulsed cathodoluminescence kinetics have different orders and are controlled by different relaxation processes.     

Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH₄H₂PO₄) and KDP (KH₂PO₄) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H⁰ centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H⁰ centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects.     

Luminescence excitation in reactive molecular CO + O<Subscript>2</Subscript> collisions on the surface of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Eu

It is shown that the CO + O₂ → CO₂ catalytic reaction on the surface of Y₂O₃-Eu may lead to electronic excitation of Eu³⁺ luminescence centers due to the chemical energy release. The luminescence observed allows one to study the interaction between molecular particles of ultralow (thermal) energies with surface by optical methods.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Cr<Superscript>3+</Superscript> spectroscopy study in ZnO-codoped and Zn-in-diffused congruent LiNbO<Subscript>3</Subscript>:Cr crystals

This paper reports on the spectroscopy properties, absorption and luminescence, of Cr³⁺ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO₃:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr³⁺ ions located in Li⁺ and Nb⁵⁺ sites; [Cr]_Li and [Cr]_Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr³⁺ ions of the two defect centres. The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO₃:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr]_Nb centres in the diffusion zone. The location of the diffused Zn²⁺ ions is considered to be in Li⁺ site, displacing the Cr³⁺ ions from the Li⁺ sites, [Cr]_Li, to the Nb⁵⁺ positions, [Cr]_Nb.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

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)     

Photodynamic processes in CaF<Subscript>2</Subscript> crystals activated by Ce<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> ions

The photochemical properties of CaF₂ crystals activated by Ce³⁺ and Yb³⁺ ions are studied. A model of the photodynamic processes induced by pumping UV or VUV radiation in active media is suggested and experimentally verified. This model explains both the presence of color centers of electronic and hole nature in crystals activated by cerium and the mechanism of suppressing of solarization processes after additional activation of the samples by Yb³⁺ ions. The cross sections of the processes of free-carrier capture by various ytterbium impurity centers are estimated. These impurity centers are established to be effective centers of recombination of free carriers of both signs.     

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.