Probing Electronic Symmetry Reduction during Charge Migration via Time-Resolved X-Ray Diffraction

The present work focuses on probing ultrafast charge migration after symmetry-breaking excitation using ultrashort laser pulses. LiCN is chosen as prototypical system because it can be oriented in the laboratory frame and it possesses optically-accessible charge transfer states at low energies. The charge migration is simulated within the hybrid time-dependent density functional theory/configuration interaction framework. Time-resolved electronic current densities and simulated time-resolved x-ray diffraction signals are used to unravel the mechanism of charge migration. Our simulations demonstrate that specific choices of laser polarization lead to a control over the symmetry of the induced charge migration. Moreover, time-resolved x-ray diffraction signals are shown to encode transient symmetry reduction at intermediate times.     

95 GHz三次谐波回旋管设计

采用线性理论和非线性理论研究了回旋管谐振腔结构、寄生模式抑制及注波互作用等问题。设计了一支工作在95 GHz的三次谐波回旋管,注波互作用结构采用标准开放式谐振单腔,工作模式为TE64, 采用电压45 kV、电流5 A、横纵速度比为1.5的小回旋电子注。在不考虑电子注速度离散及厚度的情况下,非线性理论分析表明,该回旋管可以获得14 kW功率输出,横向互作用效率约为18%,整管效率约11%。     

铅、汞、镉离子的荧光传感器研究进展

化学传感器提供了方便、快捷、廉价的分析检测有毒重金属离子的方法,并具有很高的灵敏度和选择性.它在环境科学、分析化学以及生命科学等领域有广泛的应用前景.本文分别综述了pb²⁺、Hg²⁺、Cd²⁺几种有毒重离子化学传感器的最新研究进展,并展望了该领域的发展趋势.     

Planar potentiometric sensors based on Au and Ag microelectrodes and conducting polymers for flow-cell analysis

Back-side contact Au and Ag microelectrodes were used as transducers to construct planar all-solid-state electrodes suitable for flow-through analysis. The microsensors were based on plasticized PVC potassium-selective membranes containing ion-electron conducting polymer—polypyrrole doped with di(2-ethylhexyl) sulfosuccinate. The proposed technique allowed simple construction of microsensors in one step, by membrane solution casting directly on the surface of the planar metallic transducers. The performance of the microsensors based on Au and Ag transducers were determined and compared with planar sensors based on internal electrolyte immobilized in polyHEMA. The addition of the polypyrrole to the membrane composition did not influence on the selectivity, reproducibility and long-term stability of the microsensors but improved their standard potential stability in time in comparison with coated-wire type sensors. Moreover, all-solid-state microsensors based on Au transducers exhibited better signal stability than Ag based sensors.     

Ring current and anisotropy effects on OH chemical shifts in resonance-assisted intramolecular H-bonds

Ring current effects on resonance-assisted and intramolecularly bridged hydrogen bond protons for 10-hydroxybenzo[h]quinoline 1 and a number of related compounds were calculated and the through-space NMR shieldings (TSNMRS) obtained hereby visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. These calculations revealed that this through-space effect is comparably large (up to 2?ppm) dependent on the position of the intramolecularly bridged OH proton, and therefore, contribute considerably to the chemical shift of the latter making it questionable to use δ(OH)/ppm in the estimation of intramolecular hydrogen bond strength without taking this into account. Furthermore, the anisotropy effects of additional groups on the aromatic moiety (e.g. the carbonyl group in salicylaldehyde or in o-hydroxyacetophenone of ca. 0.6?ppm deshielding) should also be considered. These through-space effects need to be taken into account when using OH chemical shifts to estimate hydrogen bond strength.     

基于半导体光放大器进行光标签提取的性能分析

利用半导体光放大器增益饱和数学模型,第一次分析了基于单个半导体放大器进行光标签提取的节点性能.通过对半导体光放大器的参量分析和优化,仿真显示,对于标签速率2.5 Gb/ s 、净荷速率40 Gb/s 的光分组包,提取的标签消光比可达到13 dB.     

Acousto-optic generation of orbital angular momentum states of light in a tapered optical fiber

We demonstrate an acousto-optic mode converter based on a tapered optical fiber to efficiently generate orbital angular momentum states of light. In our scheme an acoustic wave is deployed to the waist of tapered optical fiber where two degenerate HE₂₁ modes leading to +1 and −1 orbital angular momentum eigen-modes are resonantly excited. The excitation of TM₀₁ and TE₀₁ modes is suppressed by enlarging the intermodal index difference between near-degenerate spatial modes. Numerical calculation for optimization of the taper diameter is provided. The experimental characterization of generated states is performed by analyzing the output far-field pattern and the spatial interference fringes with a uniform reference beam.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Luminescence properties of green emission of SiO2/Zn2SiO4:Mn nanocomposite prepared by sol–gel method

Green light emitting Mn²⁺ doped Zn₂SiO₄ particles embedded in SiO₂ host matrix were synthesized by a sol–gel method. After the incorporation of ZnO:Mn nanoparticles in a silica monolith using sol–gel method with supercritical drying of ethyl alcohol in two steps, it was heat treated in air at 1200 °C for 2 h in order to obtain the SiO₂/α-Zn₂SiO₄:Mn nanocomposites. The microstructure of phosphor crystals was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD results indicate that the pure phase α-Zn₂SiO₄ with rhombohedral structure was obtained after thermal treatment at 1200 °C. The SiO₂/α-Zn₂SiO₄:Mn nanocomposites with a Mn doping concentration of 1.5 at% exhibit two broadband emissions in the visible range: a strong green emission at around 525 nm and a second one in the range between 560 and 608 nm. This nanocomposite with a Mn doping concentration of 0.05 shows the highest relative emission intensity. Upon 255 nm excitation, the luminescence decay time of the green emission of Zn₂SiO₄:Mn around 525 nm is 11 ms. The luminescence spectra at 525 nm (⁴T₁–⁶A₁) and lifetime of the excited state of Mn²⁺ ions-doped Zn₂SiO₄ nanocrystals are investigated.     

Photoinduced non-linear optical diagnostic of SiNxOy/Si111 interfaces

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in SiN_xO_y/Si1 1 1 films was proposed for contact-less monitoring of specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at wavelengths of 315, 337 and 354 nm as well as doubled frequency YAG–Nd laser wavelength (λ=530 nm). The YAG : Nd pulse laser (λ=1.06 μm; W=30 MW; τ=10–50 ps) was used to measure the PISHG. All measurements were done in a reflected light regime. We found that the output PISHG signal was sensitive to the N/O ratio and the film thickness. Measurements of the PISHG versus pumping wavelengths, powers, incident angles as well as independent measurements of the DC-electric field induced second harmonic generation indicate the major role played in this process by axially symmetric photoexcited electron–phonon states. The SiN_xO_y films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the Si1 1 1–SiN_xO_y interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of anisotropic PISHG to the N/O ratio and film thickness is revealed. The role of the electron–phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional extended X-ray absorption fine structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50.