Oxygen-phase-state changes in ZnS single crystals annealed in vapors of the constituents

Exciton reflection and emission spectra and edge emission have been investigated in ZnS sinle crystals grown from the melt and containing oxygen and then subjected to annealing in vapors of the constituents. The study of optical properties of the crystals cooled to 77°K were conducted in parallel with structural investigations of the crystals using proton and x-ray diffraction analyses. Based on the experimental data it was concluded that in the ZnS lattice, oxygen exists in a number of phase states: as part of a ZnS·O solid solution in the host lattice; as a precipitate from the saturated solid solutionβ=ZnS·O_sat; as an impurity atmosphere in the vicinity of packing faults; as ZnO precipitated on dislocations. The effect of oxygen in these various phase states on the exciton spectra and edge emission of ZnS was investigated. It was shown that oxygen is not very mobile in ZnS crystals annealed in sulfur vapor and it becomes concentrated mainly at packing defects. This leads to an increase in the concentration of packing defects and makes possible a transition from the cubic to the hexagonal modification of ZnS. The concentration of oxygen at packing defects leads to the appearance in reflection spectra and in the edge emission spectra ofα-ZnS of an additional hexagonal band located on the long wavelength side which is caused by the formation ofβ-ZnS·O solid solution. Oxygen diffuses quite rapidly through ZnS which has been annealed in zinc vapor and it precipitates from the crystals as the distinct phases ZnO orβ-ZnS · O_sat, and as a result the defect content of the hostα-ZnS lattice decreases. The concentration of the ZnO-phase is quite small and its exciton bands do not appear in the reflection spectra. The precipitation of the solid solution in the form of the phaseβ-ZnS · O_sat leads to the appearance of an additional long wavelength absorption edge in the 334 to 335 nm region (at 77°K). In addition, because single crystals of ZnS annealed in zinc vapor contain a large concentration of sulfur vacancies, there occurs a rather rapid formation of the solid solution on the layers ofα-ZnS close to the surface; this leads to a broadening and a shift toward longer wavelengths of the sphalerite exciton spectra. A similar shift is observed for the edge emission band ofα-ZnS. When the crystals are aged, theα-ZnS·O solid solution decomposes and the bands assume the standard sphalerite positions. Changes in the intensity of edge and exciton emission were investigated taking into account changes in the phase state of oxygen in the crystals.     

Exciton states in alkali halides

Recent experiments based on modulation spectroscopy have shown that it is possible to detect exciton levels in alkali halides up to n = 4. Therefore we worked out numerical calculations in order to predict the whole exciton series in KI and RbI. In our calculations the deep exciton levels are treated by considering the actual hole-electron interaction, whereas the effective mass approximation is used for the shallow exciton levels. The direct and exchange terms of the hole-electron interaction have been evaluated by performing three and four center integrals, the Wannier wave functions appearing in such integrals being approximated by suitable gaussian expansions of atomic orbitals.It is shown that by allowing the exciton state to extend up to 42 shells of neighbors it is possible to predict the exciton levels up to n = 2, the n = 3, 4 excitons being accounted for by the effective mass approximation. Similar computations performed for excitons in solid rare gases were found in excellent agreement with the experimental data and confirmed the reliability of our method.     

Electronic excitation of the matrix during drift of excess electrons through solid xenon

It is shown experimentally that the exciton luminescence λ=172 nm) quantum yield excited by excess electrons drifting through solid xenon at 77 K in fields of 10 kV/cm amounts to 20±5 per electron and that luminescence takes place during the entire drift process. A CW bulky discharge through solid xenon (with a current up to 20 A/cm²) is realized, and intense visible luminescence due to excitation of impurities by electron impacts is observed. The prospects for using solid rare gases as matrices for studying processes in low-temperature plasmas and for creating effective electric energy converters in the vacuum ultraviolet range are discussed.     

Excitons and plasmons in wide gaps: Solid rare gases

We develop a model macroscopic response function that describes the electronic response in wide gap insulators. The parameters entering the model, which consists of two coupled oscillators, are fixed by electrostatics. Application is made to both valence and core transitions in solid rare gases. We obtain exciton and plasmon energies in agreement with the available experimental data.     

铜掺杂氧化锌纳米棒的非线性光学响应竞争特性

利用飞秒脉冲激光激发Cu掺杂ZnO纳米棒,研究其特有的非线性光学性质和激发机制。在激发波长为750 nm的荧光光谱中,二次谐波峰非常弱,几乎可以忽略,存在非常强的激子发光峰和Cu掺杂导致缺陷发光峰。激发强度的增大会导致这两个发光峰强度呈非线性增大,激子发光峰位产生明显红移,而缺陷发光峰位没有变化。进一步增大激发强度,缺陷发光峰强度会出现饱和甚至有所下降,而激子发光峰强度持续增大。当激发波长增加到760 nm时,从样品的荧光光谱可以清楚地识别到二次谐波峰和激子发光峰以及缺陷发光峰并存。随着激发波长的进一步增加,二次谐波强度不断增大,而激子发光峰和缺陷发光峰的强度却随之下降。当激发波长为790 nm和800 nm时,未发现激子发光峰和缺陷发光峰,非线性光谱以二次谐波为主导。研究结果表明,通过选择合适的激发波长和激发强度,可以实现发光颜色的转变,使得Cu掺杂ZnO纳米棒在全光显示方面具有潜在的发展前景。     

Emission from evaporated anthracene films

Emission spectra, excitation spectra and decay times of fluorescence of anthracene films prepared by evaporation on to substrate cooled with liquid nitrogen were measured. The fluorescence spectra of such films show a broad structureless band. The fluorescence excitation spectra and decay times of the emission in the shorter wavelength side of the band are different from those in the longer wavelength side. The decay times of the emissions on the shorter and longer wavelength sides are about 6.0 and 190 ns, respectively, at liquid nitrogen temperature. It may be that the former emission is due to a crystalline structure and the latter emission to an amorphous structure. Next, in anthracene evaporated films containing tetracene as a dopant, the energy transfer from the host to the guest could be observed and this was attributed to the exciton diffusion through the crystalline structure.     

Influence of electron-electron scattering on the luminescence quantum efficiency in solid argon

The quantum efficiency of the intrinsic luminescence of solid Ar has been investigated for excitation energies between 15 and 55 eV. The spectral dependence of the quantum efficiency yields a stepwise increase with increasing excitation energies. The threshold energies for the steps closely correspond to the sum of the gap energy and multiples of the energy of the n = 1 exciton. The results are discussed in the framework of electron-electron scattering and give evidence that excitation of free electronic polaron complexes occurs in solid rare gases at higher excitation energies.     

非线性荧光光谱的神经网络分析及其应用

大功率超快脉冲激光和气体相互作用可产生非线性荧光光谱,不同的气体分子具有不同的非线性荧光光谱。因而这种光谱可以作为物质的指纹模式加以识别分类,进而获知气体的成分。由于不同气体分子的光谱在同一波段上有很大的交叉重叠,用传统的光谱分析方法分析存在困难,采用神经网络方法分析上述非线性荧光光谱,利用经过预处理的荧光光谱数据作为模式样本,其中一部分样本作为学习样本对级联神经网络进行训练,用训练好的网络对所有样本进行实时识别,学习样本和测试样本的的正确识别率均可达100％,结果表明此方法可实时判断混合气体的组分。     

Transient demonstration of exciton behaviours in solid state cathodoluminescence under different driving voltage

In the solid state cathodoluminescence (SSCL), organic materials were excited by hot electrons accelerated in silicon oxide (SiO₂) layer under alternating current (AC). In this paper exciton behaviours were analysed by using transient spectra under different driving voltages. The threshold voltages of SSCL and exciton ionization were obtained from the transient spectra. The recombination radiation occurred when the driving voltage went beyond the threshold voltage of exciton ionization. From the transient spectrum of two kinds of luminescence (exciton emission and recombination radiation), it was demonstrated that recombination radiation should benefit from the exciton ionization.     

Excitation spectra of the intrinsic luminescence of solid neon

The excitation spectra are measured for the atomic and the hot molecular self-trapped exciton (STE) luminescence in solid Ne. The atomic STE component is enhanced in comparison with the molecular STE at 17 and 19 eV of excitation energies, which are located at the low energy side of the bulk exciton transitions. On the other hand, the branching ratio into the atomic and the hot molecular STE's is almost constant at the other excitation energies.     

硝酸咪唑铕的OPO激光激发光谱

对新合成的固体化合物硝酸咪唑铕以及对应稀土盐的OPO激光激发光谱进行了测试,观察到在593 nm处有较弱的荧光发射而在616 nm处有较强的荧光发射,并且稀土配合物的相对荧光强度比稀土盐的相应位置的荧光强度强,分析了配合物的这种光谱特性,讨论了配体对配合物光谱的影响.此外还测试了配合物的水溶液UV/VIS光谱.     

Charged particle erosion of solid rare gases and dilute rare gas alloys

We present the first experimental results on electron-induced erosion of solid neon. The measurements are interpreted qualitatively within a new model invoking excitation transport by free excitons and their subsequent decay at the surface. The model accounts for the magnitude of the observed yield and the energy dependence. A theoretically predicted decrease in the erosion yield due to doping with a heavier rare gas, in casu argon, has been observed experimentally. The strong influence of very small amounts of different types of impurities makes sample purity a crucial problem in investigations of the erosion of solid rare gases.     

Fundamental absorption spectra of solid hydrogen

Absorption spectra of solid H₂ and D₂ are measured in the range 10–40 eV at 2K for the first time, using synchrotron radiation. The spectra can be divided into two components; exciton and interband transitions. The exciton absorption consists of bands corresponding to molecular transitions, and vibrational structures are observed in the first exciton band of solid H₂.     

A proposed ablation laser

A new type of eximer laser is proposed; it involves the simultaneous sublimation and excitation of a frozen film by means of a relativistic electron beam. Calculated beam requirements for noble gases are ～ 10 kA/cm² of 400 kV electrons for periods of 2 to 12 ns. Predicted advantages of this laser are high gain and the elimination of wavelength limiting optical windows. Preliminary experimental fluorescence spectra have been obtained from xenon films. The emission from the solid phase consists primarily of two 100 Å wide bands centered at 1690 Å and 1730 Å the fluorescence lifetimes are 4 ± 2 ns for both bands.     

Study of vibronic interactions in impurity centers by conjugate fluorescence and absorption spectra with a poorly resolved vibrational structure

The conjugate fluorescence and fluorescence excitation spectra of recently synthesized substituted arylpolyene (C₆H₅-[CH=CH]₂-C₆H₄-NH₂) are studied in solid n-octane at a temperature of 4.2 K. The spectra exhibit a weakly pronounced vibrational structure. A method of determination of the vibronic interaction parameters responsible for the shape of the spectra is developed. The method is based on the modeling of the spectra by series of vibronic bands, each of which consists of a narrow zero-phonon line and a broad phonon wing (phonon sideband). This makes it possible to calculate the fluorescence and fluorescence excitation spectra with the weakly pronounced vibrational structure and compare them with the measured spectra. The deviations from the mirror symmetry between the measured fluorescence and fluorescence excitation conjugate spectra are explained by the combined effect of the Franck-Condon and Herzberg-Teller interactions. The parameters of these interactions are determined.     

Solid state broadening in Auger spectra

Some Auger spectra in the gas phase show extremely sharp lines and a great deal of fine structure while the Auger spectra of solids, even at high resolution, lack this detailed fine structure and generally have much broader peaks. In an effort to understand the broadening processes which occur on condensation of a gas or vapour to the solid phase, several solids for which high resolution gas phase data exists have been studied. The result of measurements of the M₄₅N₄₅N₄₅ Auger spectrum of cadmium are reported and comparison is made with the vapour phase data. Although the solid data contains considerable quasi-atomic fine structure the lines are broader than the vapour phase and by matching computer broadened vapour phase data to the experimental data an estimate of the individual line widths may be made. The relative intensities of the lines in the simulation agree closely with the solid data if loss processes are considered. Measurements have also been made on the same Cd transition in CdS and in this material the Auger lines are much broader than in Cd metal and the fine structure has almost disappeared. Results are also reported of measurements of the Auger spectra of solid rare gases made with the aid of a UHV cryostat capable of operating at temperatures down to 10 K. The gases studied were Xe, Ar and Kr and again comparison with computer broadened gas data enable the broadening of individual Auger lines to be estimated. Measurements of the line breadths have been made at various temperatures and the results of these experiments will be discussed in terms of lifetime and lattice vibrational broadening.     

Magnetic-field-induced polariton effects in light reflection spectra of structures with wide exciton quantum wells

A theoretical model has been developed to describe the behavior of exciton polaritons in a wide quantum well for structures with the zinc blende symmetry in a transverse magnetic field (the Voigt geometry). The model takes into account the mixing of the 1s-1s and 1s-2p states of heavy excitons by the magnetic field and makes it possible to explain and quantitatively describe the activation of optically inactive states in the reflection spectra and the magnetic-field-induced increase in the translational mass of the exciton. The quantitative calculations of the spectra have been preformed using typical parameters for CdTe/ZnCdTe quantum-well structures.     

Development of temporal spectroscopic properties for Xerogel matrices doped with Rhodamine 6G dye

The solid host of a laser dye modifies its spectroscopic properties with respect to its liquid host. During the Sol-Gel process the dye molecules suffer from changing their environment. Two parameters affect this matter, the change in the concentration due to the evaporation of the solvent (drying) and the caging of dye molecules inside the pores or attachment to the silica network. Rhodamine 6G absorption and fluorescence spectra with different concentrations, during Sol-Gel time processing, have been studied. Both, absorption and fluorescence spectra of the dye in the solid host, for different concentrations, show a blue-shift relative to its liquid phase.     

Fluorescence properties of a composite material based on the 5CB nematic liquid crystal with gold nanoparticles

The steady-state and time-resolved fluorescence spectra of n-pentyl-n′-cyanobiphenyl and a related composite material with gold nanoparticles in the liquid crystalline and solid states have been investigated. The introduction of gold nanoparticles into the liquid crystal leads to a noticeable quenching of fluorescence in the emission region of the predimer and excimer states of cyanobiphenyl. In the solid phase, a quenching of free excitons and a short-wavelength shift of fluorescence bands have been observed.