Stimulated self-trapped exciton emission in CsI:Pb

Defects of the type of V_K and Pb⁺ centres were created in CsI:Pb under the 4.03 eV XeCl laser line irradiation at 10 K. After irradiation, the self-trapped and localized exciton emission excited by the same XeCl laser line was observed as a result of the recombination of electrons, optically released from Pb⁺, with the V_K centres. A strongly superlinear dependence of the emission intensity on the excitation intensity was found for the 3.65 eV emission of the self-trapped exciton. A much weaker superlinearity was observed for the visible localized exciton emission. Optical amplification of the exciton emission was considered as the most probable reason of the observed phenomenon. At 10 K, optical gain G=3.74 was calculated for the self-trapped exciton emission.     

Low-temperature X-ray and photoluminescence of lead halide crystals

The emission spectra of PbF₂, PbCl₂ and PbBr₂ monocrystals are measured under optical and X-ray excitation at liquid helium temperature. Certain emission bands are attributed to the radiative transitions in the self-trapped cation exciton (excited Pb²⁺ion). The increase of trapped exciton via electron-hole recombination is discussed.     

The hopping motion of the self-trapped exciton in NaCl

The decay kinetics and the yield of the π luminescence from the lowest triplet state of the self-trapped exciton have been studied in NaCl containing Li⁺ ions. It is found that the π luminescence band which is observed at 6K is replaced by a luminescence band peaked at 3.34 eV above 77K. The 3.34 eV luminescence band is ascribed to the recombination of the relaxed exciton trapped by a Li⁺ ion, (V_ke)_Li. The decay of the π luminescence induced by an electron pulse and the time change of the luminescence from (V_ke)_Li are explained in terms of the characteristic equation of the diffusion-limited reaction of the lowest triplet self-trapped excitons with the Li⁺ ions. From the analysis of the dependence of the decay rate of the π luminescence on temperature and on the Li⁺ concentration, we found the diffusion constant D of the lowest triplet self-trapped exciton in NaCl to be given by with $\text{D}{}_0\text{= 2.13 × 10}{}^{\mathrm{?3}}\text{cm}{}^2\text{s}$ and E₀ = 0.13 eV. The present result can be regarded as the first clear experimental evidence for the hopping diffusion of the self-trapped exciton in alkali halides. The obtained values of E_a and D₀ are discussed using the small polaron theory. The effect of the anharmonicity on the hopping of the self-trapped excitons is suggested to be significant.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

YAG:Pr3+的真空紫外光谱分析及其4f5d能级的研究

用共沉淀法制备出不同掺杂浓度的钇铝石榴石(Y₃Al₅O_{12，YAG)：Pr³⁺粉末.在不同温度下用同步辐射的真空紫外光(VUV)作为激发光 源测量了其发射和激发光谱.对YAG：Pr³⁺被VUV激发后的发光过程进行了分析， 并估算了YAG：Pr³⁺的声子能量、黄昆因子S及Stokes位移.当用170nm光激发样 品时，可能存在自陷激子和通过自陷激子能量传递引发的³P0发射 ，这种跃迁途径不同于用240nm和289nm两个波长激发后的跃迁途径；对Pr³⁺在Y AG中的4f5d能级进行了研究，从而对其4f5d组态有了一个较清晰的认识. 关键词： 3+')" href="#">YAG：Pr³⁺ 同步辐射 4f5d能级 VUV}     

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.     

C60中非线性激发的动力学研究

用电子－晶格耦合的紧束缚模型和求解实时牛顿动力学方程的方法研究了Ｃ₆₀受光激发后的动力学过程，得到了受光激发后Ｃ₆₀分子的能量，键结构和电子状态的动态演化，结果表明光激发Ｃ₆₀分子演化成环状双极化子激子。该计算结果与中性富勒烯材料的荧光实验所得出的结论一致。 关键词：     

Theoretical investigations on the high light yield of the LuI3:Ce scintillator

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of at least three factors controlling the energy transfer from the host matrix to activator. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI₃, and inefficient in LuCl₃. To justify this channel, we perform calculations of density of states using a periodic plane-wave density functional approach. The second factor is the increase of the efficiency of valence hole capture by cerium in the row LuCl₃-LuBr₃-LuI₃. The third one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the same row. The latter two factors are verified by cluster ab initio calculations. We estimate either the relaxation of these excitations and barriers for the diffusion of self-trapped holes (STH) and self-trapped exciton (STE). The performed estimations theoretically justify the high LuI₃:Ce³⁺ scintillator yield.     

Resonance photoconductivity in Fe-doped InP

A photoconductivity peak at 0.44 eV is observed in Fe-doped InP. This peak is attributed to electrons which are optically excited from the Fe²⁺⁵E ground state to vibronic levels of the ⁵T₂ excited state, and thermally excited from there to the conduction band. The lineshape is nearly Gaussian and is well fitted by assuming a vibronic coupling energy of about 0.06 eV in the ⁵T₂ state. In analogy with results in the II–VI compounds this coupling may be dominated by interaction with non-symmetric lattice modes which produce a dynamic Jahn-Teller effect.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Molecular coherence effects in stimulated raman scattering

A semi-classical calculation of the three-level system consisting of the ground state, the vibrationally excited state and the electronic excited state under the laser and the Stokes perturbation is given. The induced molecular polarization produces gain modulation of the Stokes and loss modulation of the laser at a frequency that is dependent on the optical intensity. With the optical intensity in self-trapped filaments in nonlinear liquids such as CS₂, the period of modulation becomes of the order 10^?11 s and a large amplitude modulation of the laser and the Stokes waves will result. The amplitude modulation is not much reduced, if the molecular relaxation time of the order 10^?11 s is taken into account. Effects of non-uniform field distribution and the width and shape of the incident laser pulse are discussed. The frequency broadening caused by the three-level effect is shown to be larger than, or at least as large as, the broadening caused by the optical Kerr effect.     

Broadband infrared emission of erbium–thulium-codoped calcium boroaluminate glasses

The near-infrared emission from calcium boroaluminate (CABAL) glasses codoped with Er³⁺ and Tm³⁺ has been investigated. It is shown that by controlling the [Tm]/[Er] concentration ratio a fairly flat emission with a bandwidth of 370 nm can be achieved in the wavelength range from 1.4 to 2.0 μm. The broadband emission is formed by three bands centered at 1.4, 1.5 and 1.8 μm, which are related to the emission from the Tm³⁺: ³ H ₄→³ F ₄, Er³⁺: ⁴ I _13/2→⁴ I _15/2 and Tm³⁺: ³ F ₄→³ H ₆ transitions, respectively. Compared to Er-only doping and Tm-only doping at the same concentration, codoping with both ions leads to a reduction of the intensity and lifetime of the Er-related band at 1.53 μm and to an intensity enhancement of the two Tm-related emission bands at 1.46 and 1.80 μm. This is a result of energy transfer (ET) processes between Er³⁺ and Tm³⁺ ions that are relevant when determining the emission spectrum shape. Two dominant ET processes are identified, both consisting of transferring the energy of the Er³⁺ first excited level (⁴ I _13/2) in the first case to the Tm³⁺ first excited level (³ F ₄), which is excited to the third excited level (³ H ₄), and in the second case to the Tm³⁺ ground state (³ H ₆) which is excited to the first excited level (³ F ₄).     

Electronic Raman spectra of iron doped MgO

The nearby excited states of Fe²⁺ in MgO are investigated by Raman spectroscopy. We observed an A_1g impurity mode (185 cm^?1) and an electronic transition at 110.5 ± 0.8 cm^?1 which we associate with the first excited states of the ferrous ion, Γ₃ and Γ₄, previously observed by far infrared optical absorption.     

Self-trapped exciton in alkali fluorides: ODMR study

The optical detection technique is used to observe magnetic resonance in the triplet state ³B_1u of the self-trapped exciton in LiF, NaF, KF. The strong anisotropy of the hyperfine interaction in the x?y plane is related to the more localized character of the electronic wavefunction. The magnetic dipolar contribution to the fine structure can be described by a Mollwo-Ivey type law in alkali fluorides and chlorides.     

Luminescence of impurity-bound excitons in corundum

The luminescence characteristics of M ³⁺: Al₂O₃ crystals, where M ³⁺ stands for an isoelectronic cation impurity with a filled electron shell, namely, Sc³⁺, Y³⁺, or La³⁺, were studied. The luminescence of excitons bound (BE) to these impurities was detected. The position of the BE energy states at the long-wavelength absorption edge as a function of the M ³⁺ ionic radius was established. The energies of the long-wavelength BE creation threshold and of the maximum of the BE luminescence band were approximated empirically by third-order polynomials of the Toyozawa polynomial type, which describes electron-phonon interaction. The energy and spatial structure of the BE was found to be similar to that of a self-trapped exciton (STE). The BE and STE states are separated by an energy barrier, and energy transfer from the STE to BE is frozen at low temperatures.     

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)     

Spectral luminescence properties of Ca1 ? x Sr x F2:Ce3+ (0 < x <1) crystals

The luminescence, reflection, and luminescence excitation spectra of two-component Ca_{1 ? x} Sr _x F₂:Ce³⁺ (0.05 mol %) (x = 0.14, 0.25, 0.4, 0.6, and 0.75) have been studied at room temperature and T = 8 K. It is shown that the luminescence bands (upon 130-eV photon excitation) in the range of 200 to 400 nm are attributed to singlet and triplet self-trapped exciton luminescence and to 5d-4f transitions in Ce³⁺.     

Relaxation, self-trapping, and decay of electron excitations in wide-gap oxides

Emission spectra ofMgO, Al ₂O₃, Y₂O₃, andSc ₂O₃ crystals, excited at 8 K by photons with energies 5–40 eV, are measured. The luminescence of free excitons andGe ²⁺ centers forMgO, the self-trapped exciton emission (STEE) forAl ₂O₃ andY ₂O₃, and the tunnel self-luminescence forScO ₃ are studied. A distinction between hole polarons in oxides and self-trapped holes in halides leads to a sharp difference in the mechanisms of STEE creation on electronhole recombination for these two classes of solids. The electron-hole, hole-electron, and exciton mechanisms of excitation multiplication are identified. The high radioresistance of wide-gap oxides and the behavior of oxygen interstitials inMgO crystals are discussed. Institute of Physics of Estonia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 5–16, March, 2000.