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.     

Electronic structure of the self-trapped exciton in the potassium halides

We present pseudopotential calculations of the energy levels and wave-functions of the electron associated with the self-trapped exciton in KCl, KBr and KI. Accurate values are obtained for the separation of the singlet and triplet luminescent bands, for the electronic transitions, and for the spin resonance linewidths. Lattice distortion is shown to be particularly important in KCl. Additional low-energy electronic transitions, at 0.7 eV in KCl and 0.94 eV in KBr, are predicted, but have not been seen experimentally.     

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.     

Electronic structure of the self-trapped exciton in rare gas solids

The Rydberg-like series of the self-trapped exciton R₂¹ in rare gas solids (Ar, Kr and Xe) are obtained by solving the effective mass equation which incorporates different corrections, including the central cell correction. The results are in good agreement with the recent transient optical absorption data in which the electron is excited into higher excited states. The origin of the luminescence bands is interpreted by analogy with a similar structure of the self-trapped excitons in alkali halides.     

Absorption spectra of self-trapped exciton in condensed rare gases

Optical absorption from the self-trapped exciton (STE) to its higher excited states has been observed in solid Ar, Kr, and Xe. A new absorption peak of Ar1₂ is found. Comparison of solid, liquid and gas phase data provides evidence of the similarity of level structures of Ar1₂ and STE. Tentative assignments of the observed transitions are also made.     

KMgF3晶体的色心和自陷态激子研究

应用从头计算方法对KMgE3离子晶体中的色心和自陷态激子(STE)进行了模拟研究.对包含F心的量子团簇进行几何结构优化,并计算了F心处于1s基态和2p激发态的Mulliken电荷分布.模拟结果表明,F心周围的晶格弛豫较小,处于基态的F心电子主要局域在阴离子空位处,而处于激发态的F心电子波函数则比较扩展.计算结果表明,Vk心移向邻近的间隙位置,但仍保持分子轴向与[110]晶向平行.自陷态激子的弛豫是分子轴向平移与转动的叠加.计算得到的F心、Vk心的光学激发能及STE的发光能与实验符合得较好.     

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.     

Luminescence of exciton in GeS

The luminescence of one free and four bound excitons has been investigated in the layer compound GeS. Up to six phonon replicas of the bound excitons are observable. The energies of these phonons are in reasonable agreement with Raman- and IR reflectivity measurements. The luminescence is highly anisotropic and is only seen in the polarization E ∥ a which confirms that the transitions E ∥ b are forbidden. Measurements with E ∥ c were inconclusive because of the difficulties in the sample preparation. Time resolved measurements revealed decay times between 20 nsec and 1.5 μsec for the different bound excitons. One exciton has been identified as bound to a Si impurity. This is the first observation of intentional doping of GeS. Thermal dissociation energies for the bound excitons lie between 6.5 and 20 meV.     

Selective non-radiative transitions at excited states of the self-trapped exciton in alkali halides

Generation of F-H pairs and σ-luminescence induced by excitation of the self-trapped excitons with polarized light causing the 1s→ 2p transitions has been measured. The results were analyzed based on the assumption that the non-radiative transitions that follow photoexcitation depend only on the state reached by the excitation, irrespective of the photon energy of the excitation. The absence of the dependence on exciting photon energy of the effective yield of removal from the triplet manifold after excitation to each substate obtained from the analysis proves that the above assumption is valid. The relative non-radiative transition probabilities between the 2p substates and from the 2p-substates to the lowest triplet state, the F-H pair, the σ-luminescent state and the ground state were obtained. It is shown that the de-excitation channels from each substate are substantially different from each other. The following transitions are found to have high probabilities: from the B_1g state to the F-H pairs in KCl and KBr, from theA_g state to the σ-luminescent state and the lowest triplet state in KBr and to the lowest triplet state in KCl and from the B_2g state to the B_1g state in KCl and KBr, where A_g, B_1g and B_2g denote the states with the electron excited to the σ_u orbital, the π_u orbital lying in the (100) plane in which the (halogen)₂^?-molecular ion is situated and the other π_u orbital, respectively. The mechanisms of these non-radiative transitions were discussed.     

Luminescence of Ca(NbO3)2:Pr3+: Pr3+ and self-trapped exciton emission

Photoluminescence and time resolved photoluminescence spectra of Ca(NbO₃)₂ doped with Pr³⁺, excited under 37,000 cm^?1 (270 nm), obtained at high hydrostatic pressure up to 20 kbar applied in a sapphire anvil cells, are presented. At ambient conditions, the emission spectrum obtained in the time interval 0–1 μs is dominated by spin allowed transitions from the ³P₀ state. The luminescence related to transitions from ¹D₂, characterized by a decay time equal to 33 μs, is observed when one excites directly the Pr³⁺ ion with 30,770 cm^?1 (325 nm) wavelength. The introduction of Pr³⁺ impurities in Ca(NbO₃)₂ does not quench the self-trapped exciton (STE) luminescence. This luminescence, peaking at 20,000 cm^?1 (500 nm), having a decay time of 61 ± 1 μs, still occurs when the crystal is excited with a wavelength of 37,000 cm^?1 (270 nm) or shorter. Under such excitation a fraction of the STE luminescence is reabsorbed by Pr³⁺ ions; in this case the emission lifetime of the ¹D₂ → ³H₄ transition of Pr³⁺ is 64 ± 3 μs. This effect is stable also at high pressure.     

Radiative and nonradiative recombination of self-trapped exciton on silicon nanocrystal interface

The theory of the multiphonon and radiative recombination of a self-trapped exciton on the interface of a silicon nanocrystal in a SiO₂ matrix is developed. Self-trapped excitons play a key role in the hot carrier dynamics in nanocrystals under photoexcitation. The ratio of the probabilities of the multiphonon and radiative recombination of the self-trapped exciton is estimated. The probabilities of exciton tunnel transition from the self-trapped state to a nanocrystal are calculated for nanocrystals of various sizes. The infrared range spectrum of the luminescence of the self-trapped exciton is obtained.     

Luminescence of self-trapped excitons in calcium fluoride under pulsed electron irradiation

Time-resolved pulsed spectroscopy was used to measure the luminescence spectra of calcium fluoride. Characteristic features of the luminescence of self-trapped excitons are discussed. It is shown that various configurations of self-trapped excitons incorporating hole nuclei of a more complex structure, may be formed in CaF₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 1226–1227 (July 1998)     

Deformation Luminescence in gamma-irradiated KCl single crystals

Abstract

Deformation Luminescence(DL) is studied on KCl crystal colored by γ-irradiation. The spectra analysis of DL and thermoluminescence reveals that F-center plays a role as electron donor through the interaction with moving dislocation and V₂-center is a probable luminescence center. A theory is presented for understanding the deformation rate and temperature dependences of DL intensity.