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.     

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.     

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.     

KMgF3晶体的色心和自陷态激子研究

应用从头计算方法对KMgF3₃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_K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关键词： 色心 自陷态激子 从头计算 3离子晶体')" href="#">KMgF3₃离子晶体     

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.     

真空紫外光激发下掺Pr3+铝酸盐的自陷激子对Pr3+发光的影响

研究了SrAl12 O19:Pr3+和Y3Al5O12(YAG):Pr3+在20K,77K和300K的真空紫外(VUV)光谱,二者除了表现出Pr3+特有的发射性质外,还产生了一种特殊的发射现象,即自陷激子(STE)发射,并且其随着稀土离子浓度和温度的降低而增强.还着重讨论了自陷激子对Pr3+发光的影响.     

The influence of exciton motion on spin-resonance absorption

We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ₁) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.     

Measurement of interchain and intrachain exciton hopping barriers in luminescent polymer

The integrated photoluminescence intensity in thin films of 'Super Yellow' copolymer has been analyzed using a Mott-like temperature dependence. This has enabled us to observe contributions from two emission channels, indicative of exciton recombination proceeding from two distinct origins. At high temperature, interchain thermally activated exciton energy transfer and migration dominates, resulting in large scale quenching of the integrated emission intensity and hence the photoluminescence quantum yield. However, at relatively low temperature, an additional increase of the integrated emission intensity occurs. This new channel of emission has been attributed to recombination from excitons where intrachain exciton energy transfer between adjacent subunits of the copolymer backbone becomes hindered. The activation energy barriers that control both of these emission channels have been obtained and are correlated with chain backbone degrees of freedom.