Energy levels and optical oscillator strengths of inner-shell excited states and photoionizations of the ground and first excited states of C IV ion

Extensive configuration interaction wave functions are determined to calculate the energies of the inner-shell excited states and the oscillator strengths of the optically allowed inner-shell transitions of C IV ion. Photoionization cross-sections of the ground and the first excited states of C IV ion are also obtained by using the R-matrix method. The positions of some inner-shell excited states are redetermined more accurately by analyzing the resonance structures of the photoionization processes. Some of the results are compared with other available theories and experiments. Received 29 September 1999 and Received in final form 8 December 1999     

Calculation of absolute electron-impact ionization cross-sections of dimers and trimers

We report calculations of the electron-impact ionization cross-sections of selected dimers (homonuclear diatomic molecules) and trimers (homonuclear triatomic molecules) using a method which relies only on macroscopic quantities in conjunction with a “defect concept”. The empirically determined defect describes the deviation of the cluster (dimer, trimer) cross-sections from a simple linear dependence on the cluster size. We compare the calculated cross-sections to experimental data for the dimers S₂ and F₂ and the trimer O₃ and we present predictions for the ionization cross-sections of Br₂, I₂, C₂ and C₃ for which no experimental data are available. Lastly, we extend the method to the calculation of ionization cross-sections for the fullerenes C₆₀ and C₇₀. Received 6 December 1999 and Received in final form 10 April 2000     

Vibrational excitation by positron collisions with molecular gases: a study of O and NO targets

The scattering of slow positrons from and NO molecules is treated using exact static interactions and a model potential for correlation-polarisation forces. The quantum coupled equations for the elastic scattering are extended to vibrationally inelastic processes and the different excitation probabilities are evaluated. Comparison with existing experiments for the NO target indicates that the present calculations provide a realistic treatment of positron scattering below Ps formation and give computational estimates on the efficiency of such projectiles in producing vibrationally excited molecules in the ambient gas. Received: 23 April 1999 / Received in final form: 3 June 1999     

Radiation-induced color centers in the near-surface layer of lithium fluoride crystals

For radiation-induced intrinsic color centers, we show that the concentrations of identical centers and the concentration ratios of different centers are quite different in the near-surface layer and within the interior volume of a lithium fluoride crystal. We have established that these differences also depend on the sign of the difference between the temperature at which the crystal was irradiated with γ photons and the vacancy mobility temperature. We provide an interpretation for the results obtained, based on the structural features of the near-surface layer and the concentration ratio of vacancies and electrons in the layer, serving as the starting components for color center formation. We found that the concentrations of centers change over the course of a few days by tens of percent in the layer “emerging” from the interior onto the surface as a result of cleavage of the crystal. We measured the luminescence lifetimes of F₃ (R2) and F₄ centers: 6.6 nsec and 11.7 nsec. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 775–781, November–December, 2006.     

Time-resolved luminescence from annealed synthetic quartz under 525 nm pulsed green light stimulation

The influence of annealing on lifetimes has been studied in synthetic quartz annealed at temperatures up to 800°C using time-resolved optical stimulation in the range 20–200°C. Luminescence was stimulated using pulsed 525 nm light-emitting-diodes. There is an increase with annealing temperature in the lifetime measured at 20°C for samples annealed beyond 500°C. Further, lifetimes decrease with measurement temperature in a manner consistent with the thermal quenching of luminescence.     

The excitation energies,ionization potentials and oscillator strengths of neutral and ionized species of Uub (Z = 112) and the homologue elements Zn,Cd and Hg

Multi-configuration Dirac-Fock method (MCDF) is employed to calculate excitation energies, ionization potentials and oscillator strengths for all neutral and up to 5 times ionized species of element Uub, as well as the homologue elements Zn, Cd and Hg. On the basis of not too extended MCDF calculations, we studied some peculiar properties of element Uub resulting from its stronger relativistic and electron correlation effects. Using an extrapolative scheme, improved ionization potentials of Uub were obtained with an uncertainty of less than 0.5 eV. Furthermore, we calculated the low-lying resonance excitation energies, absorption oscillator strengths and the first ionization potential for Hg and Uub using large scale MCDF calculations, which improved the uncertainty of the excitation energies to less than 0.25 eV for element Hg. We hope that such calculations yield good results for element Uub.     

Synthesis and luminescence properties of nanocrystalline Gd2O3:Eu by combustion process

The nanocrystalline Gd₂O₃:Eu³⁺ powders with cubic phase were prepared by a combustion method in the presence of urea and glycol. The effects of the annealing temperature on the crystallization and luminescence properties were studied. The results of XRD show pure phase can be obtained, the average crystallite size could be calculated as 7, 8, 15, and 23 nm for the precursor and samples annealed at 600, 700 and 800 °C, respectively, which coincided with the results from TEM images. The emission intensity, host absorption and charge transfer band intensity increased with increasing the temperature. The slightly broad emission peak at 610 nm for smaller particles can be observed. The ratio of host absorption to O²⁻-Eu³⁺ charge transfer band of smaller nanoparticles is much stronger compared with that for larger nanoparticles, furthermore, the luminescence lifetimes of nanoparticles increased with increasing particles size. The effects of doping concentration of Eu³⁺ on luminescence lifetimes and intensities were also discussed. The samples exhibited a higher quenching concentration of Eu³⁺, and luminescence lifetimes of nanoparticles are related to annealing temperature of samples and the doping concentration of Eu³⁺ ions.     

Two-color pump-probe experiments in helium using high-order harmonics

A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p ¹P and 1s3p ¹P states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of τ(1s2p) = 0.57 ns and τ(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6×10^-5 mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p ¹P and He 1s3p ¹P states in the threshold region are determined by measuring the saturation of the ionization process with a precision of ∼ 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections. Received 3 June 2002 / Received in final form 14 August 2002 Published online 22 October 2002 RID="a" ID="a"Present address: Department of Biophysics, Leiden University, 2333 CA Leiden, The Netherlands. RID="b" ID="b"Present address: Continuum Electro-Optics Inc., 3150 Central Expressway, Santa Clara, CA 95051, USA. RID="c" ID="c"Present address: CEA/DRECAM/SPAM, CEN Saclay, 91105 Gif-sur-Yvette, France. RID="d" ID="d"Present address: Department of Physics, National University of Rwanda, Butare, Rwanda. RID="e" ID="e"Present address: CELIA, Université Bordeaux 1, 33405 Talence, France. RID="f" ID="f"Present address: MAX-Lab, Lund University, Box 118, 221 00 Lund, Sweden. RID="g" ID="g"e-mail: anne.lhuillier@fysik.lth.se     

M/G1/1型可修排队的强度守恒律分析

用从平稳点过程和Palm分布理论推得的强度守恒律尝试研究了寿命为一般分布的M／G1／1型可修排队系统,在求得模型稳态工作量和拟虚等待时间表达式的基础上,得到了服务台的首次故障前时间,系统可用度,平均失效概率,服务台平均失效次数和系统故障频度等．有趣的是,当寿命分布取其特例指数分布时,与文选中已知的结果完全一致．