Picosecond excitonic luminescence in ZnO and other wide-gap semiconductors

Radiative lifetimes of free-atom transitions, scaled by ω³ for comparison at 368 nm, are not faster than about 6.9 ns. BaF₂ core-valence luminescence, scaled in the same way from 220 to 368 nm, corresponds to 4.1 ns. In contrast, excitonic transitions in wide-gap semiconductors display subnanosecond radiative lifetimes, and in particular ZnO has radiative lifetimes measured at 50–300 ps for D^o,X and 400–900 ps for free excitons. The “giant oscillator strength” corresponding to these lifetimes can be explained by theories developed initially for defect-bound excitons, then quantum wells, and nanoparticles. An exciton is a coherent array of N dipoles, where N is the number of sites covered by coherent translational motion of the exciton. This is not essentially a phenomenon of multiple excitons, but applies as well to single-exciton decay. It differs in that regard from the more familiar Dicke giant dipole of N coherently excited atoms lacking translational periodicity. The phenomenon suggests possibilities for achieving ultrafast scintillators and high light yield.     

光纤制导关键技术分析和研究

系统地讨论、分析和研究光纤制导中的关键技术，并对未来的发展提出了建议。     

类锂硅离子能级结构与振子强度的理论计算（相对论DF模型）

用相对论性的ＤＦ方法与非相对论性的ＨＦ方法分别计算了类锂硅离子能级结构及振子强度、跃迁概率，考查了计算精度，对数值结果进行了理论分析并与其它结果进行了比较。     

高强度均匀静磁场中氢原子能级的计算

采用一种简便的变分法，计算了在均匀高强度静磁场下氢原子的１１个低能态能级和平均半径，讨论了高强度磁场对能级和半径的影响。     

Beta decay rates for nuclei with 115<A<140 for r-process nucleosynthesis

For r-process nucleosynthesis the β-decay rates for a number of neutron-rich intermediate heavy nuclei are calculated. The model for the β-strength function is able to reproduce the observed half-lives quite well.     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON''S RATIO

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete (NC) and high strength concrete (HSC) in compressioncompression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 Mpa. The conception of damage Poisson's ratio is defined and the expression for damage Poisson's ratio is determined basically.The failure mechanism of concrete is illustrated, which points out that damage Poisson's ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.     

The steering of molecules in simple dissociation reactions

The effectiveness of steering in simple dissociation reactions has been studied by performing classical and quantum simulations on a wide range of model potentials. By parametrically varying the PES landscape, it is shown how trends in the translational energy dependence of the dissociation probability may be related to particular topographical features. The relationship between the strength of attractive and repulsive regions determines the force which a molecule feels, but in addition we show that the range of the force helps determine the degree of repositioning and reorientation. The presence of wells along the reaction coordinate is considered, and it is shown how the snarled trajectories which frequently result relate to precursor-like behaviour even in the absence of dissipative processes.     

STATISTICALLY FRACTAL STRENGTH THEORY FOR BRITTLE MATERIALS

Based on the hypothesis of the fractal distribution of crack sizes in brittle materials andthe weakest link principle,the relationship between the fractal dimension of the size-frequency distri-bution of cracks and the Weibull Modulus is derived,which reveals the geometrical nature of theWeibull Modulus.The influences of the size distribution and the orientation distribution of cracks aswell as the irregularity of the crack propagation on the strength are all taken into account.Finally,ageneral expression for the statistical strength of brittle materials in complex tensile stress state is ob-tained.     

Systematic trends in the line strengths of E1 transitions in the oxygen isoelectronic sequence

Line strengthsS for the dipole allowed transitions within then=2 complex of the oxygen isoelectronic sequence have been fitted in the formZ ² S=A+B/(Z −C), whereZ is the nuclear charge of a particular ion. The constantsA, B andC are determined by using a non-linear least square method. The data forS are taken from the configuration interaction calculations which included internal, semi-internal and all external type correlations for ions in the rangeZ=8 − 25. It is shown that the values ofA obtained from the fit for all the transitions are in excellent accord with the ab-initio values obtained in the hydrogenic limitZ → ∞ provided near degeneracy effects are included in the ground state multiplet 1s²2s²2p^{4 1}S.     

Quantum-chemical calculations of O-O bond strengths in organic hydrotrioxides

The O-O bond strengths in ten organic hydrotrioxides have been calculated by semiempirical MNDO and AMI methods. The RO-OOH bond strength is independent of the nature of substituent R and is equal to 20.4±1.1 kcal mol^–1 (AM1). The influence of the inductive effect of substituent R on the value ofD(ROO-OH) has been established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1129–1131, May, 1996.