制备工艺对电子俘获材料性能的影响

利用等离子体质谱(ICP-MS)技术测试了硫化助熔剂法和稀土直接掺杂工艺合成电子俘获材料的化学计量比,结果表明,采用稀土直接掺杂工艺合成材料的化学计量比更接近设计值;通过分析电子俘获材料的发光机制和光谱测试结果,讨论了制备工艺条件对电子俘获材料中稀土离子化合价的影响.     

Ellipsometric spectroscopy study of photoluminescent Si/SiO2 systems obtained by magnetron co-sputtering

Si nanograins embedded in silica matrix were obtained by magnetron cosputtering of both Si and SiO₂ at different substrate temperature (200–700°C) and thermal annealing at 1100°C. The samples were characterized by ellipsometric spectroscopy, high-resolution electron microscopy observations and photoluminescence. The highest excess of Si atoms was found to be incorporated for deposition temperature near 400–500°C, giving rise to a maximum PL and a shift of the peak position towards lower energy. These features might be interpreted in terms of quantum size effects and of density of grains, even though the interface states seem to be involved in the improvement of the photoluminescence efficiency.     

Interband collective electronic excitations in resonant Raman scattering of two-subband quantum wires

Using the bosonization technique, a theory for the collective excitations of the interacting electrons in quantum wires with two subbands occupied is developed. The dispersion relations for the inter-subband charge and spin density excitations are determined. The results are used to interpret the features observed in recent measurements of the Raman spectra of AlGaAs/GaAs quantum wires, particularly for photon energies near band gap resonance. It is shown that peaks previously identified as “single particle excitations” are signatures of higher order collective spin density excitations. Predictions about the observability of the interband modes are made. Received 8 February 1999     

Experimental determination and modelization of a unique phase diagram: solid solutions of antiferromagnetic organometallic radical cation salts

The physical properties of paramagnetic Mo(V) organometallic radical cation salts, , are investigated through the study of two series of solid solutions incorporating two kinds of anions ( or ). The combination of EPR and X-ray diffraction is used to specify the nature of the structural phase transitions which occur in the paramagnetic phase and to determine the corresponding ( T , x ) phase diagrams. Finally, antiferromagnetic resonance is studied to probe the low temperature antiferromagnetic ground state. In the last part of the paper, the observed complex phase diagram is analyzed by considering a compressible model with at least two independent modes of compression. We conclude that the larger compressibilities are associated with the smaller anions. Received 5 August 1998     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Coherent resonant transport through a mesoscopic system with quantum ac microwave field

The time-dependent transport through an ultrasmall quantum dot coupling to two electron reservoirs is investigated. The quantum dot is perturbed by a quantum microwave field (QMF) through gate. The tunneling current formulae are obtained by taking expectation values over coherent state (CS), and SU(1,1) CS. We derive the transport formulae at low temperature by employing the nonequilibrium Green function technique. The currents exhibit coherent behaviors which are strongly associated with the applied QMF. The time-dependent currents appear compound effects of resonant tunneling and time-oscillating evolution. The time-averaged current and differential conductance are calculated, which manifest photon-assisted behaviors. Numerical calculations reveal the similar properties as those in classical microwave field (CMF) perturbed system for the situations concerning CS and squeezed vacuum SU(1,1) CS. But for other squeezed SU(1,1) CS, the tunneling behavior is quite different from the system perturbed by a single CMF through gate. Due to the quantum signal perturbation, the measurable quantities fluctuate fiercely. Received 28 May 1998     

Dielectric-Lined Multiwave Cerenkov Generators Producing High Power Millimeter Waves

This paper presents the concept of a Dielectric-lined Multiwave Cerenkov Generator(DMWCG) producing high power millimeter waves, which is investigated with a two and one half dimensional( ) electromagnetic relativistic Particle-in-Cell(PIC) simulation code. It is showed that the DMWCG can operate in a lower diode-voltage regime with much higher radiation efficiency as compared with the usual Multiwave Cerenkov Generator(MWCG). The simulation work indicates both the downshift of the wave frequency in the presence of the dielectric liner and the existence of the optima for the permittivity of the liner as well as for the magnitude of the guiding magnetic field. The required intensity of the guiding field is reduced with the introducing of the liner. The radiation is generated at the dominant frequency of 31.5GHz. The power level of 1.5GW is achieved, with radiation efficiency up to 15%. The features of parameter dependency are presented. And reasonable explanation is put forward. In addition, the enhanced propagation of the electron beam is studied in the presence of the dielectric liner.     

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为了研究靶材料对快电子能量分布的影响,采用电子谱仪测量了飞秒激光与Cu和CH靶相互作用中在靶前和靶后产生的快电子能谱。结果显示,在靶前Cu和CH靶的快电子能谱相似,反应了快电子发射对靶材料的依赖性较弱;在靶后Cu和CH靶的快电子能谱具有明显的差异,说明电子的输运过程与靶材料密切相关。冷电子环流以及自生磁场是导致Cu靶快电子能谱"软化"的原因,而对于CH靶麦克斯韦分布的快电子能谱主要由碰撞机制决定。     

黄素体系电子转移过程光谱研究

黄素类物质在生物体内广泛存在,是许多电子转移反应的活性中心,也是电子传递链的重要组成部分。其受到光照激发后引起的电子转移,是许多生命过程的基础与起始步骤。特别地,一种名为隐花色素的黄素蛋白在光激发后经一系列电子转移形成自旋相关自由基对,被认为是最有可能的生物磁敏物质,更使黄素体系电子转移过程的动力学,特别是自旋动力学过程倍受关注。对黄素电子转移过程及相关机理进行研究,有助于厘清多种生命过程的化学机理与影响因素。为此,科学界综合运用了多种仪器与测试手段,其中主要包括紫外-可见光谱,荧光光谱,瞬态吸收光谱,光化学诱导动态核极化（Photo-CIDNP）技术等。通过多年的研究,对黄素在生物体内的作用机理与电子转移过程的认识经历了由浅入深,不断深入的过程。紫外-可见光谱（UV-Vis）主要用于研究黄素系统中的电子激发,自旋动力学和电子转移。结合理论计算,UV-Vis还可以识别电子转移中涉及的基团并进行定量分析。荧光光谱可以识别电子受激发的物质,在反应过程中观察黄素和半醌中间体的产生和消耗,并确定其氧化还原和质子化状态。瞬态吸收光谱适于观测反应过程中出现的短寿命物种,其中飞秒泵浦探测技术的引入大大提高了观测的时间分辨率,并且可以通过光谱特征区分单重态和三重态的自由基对。光化学诱导动态核极化核磁共振（NMR）可以直接观察电子-核自旋动力学过程。磁场依赖性photo-CIDNP NMR揭示了控制单重态与三重态互变的因素,并提出了生物地磁导航可能依赖的化学机制。腔吸收与单分子光谱的运用,从技术上提高了实验装置的灵敏度并降低检测限。主要介绍黄素体系电子转移过程研究所运用的各种光谱手段与取得的成果,并展望其未来。     

EBSD-XPS法分析磷石膏中杂质物相

磷石膏是湿法磷酸过程形成的固体副产物。磷石膏中含有磷、氟、硅等有害杂质组分,极大影响磷石膏制品的质量和性能,巨量磷石膏堆存严重威胁生态环境和生命安全。确定磷石膏中杂质物相的赋存状态,为磷石膏除杂净化和综合利用提供理论指导,非常重要。以低温干燥后的磷石膏为研究对象,利用X射线荧光光谱（XRF）分析确定磷石膏中杂质元素的组成,结果表明,磷石膏中的杂质元素含量较高的有P,Si,F和Al,含量较低的有Ba,Fe和Mg等。因二水硫酸钙物相强峰对杂质物相峰有较强遮蔽作用,X射线衍射光谱（XRD）分析不能确定磷石膏杂质的物相。利用扫描电子显微镜对磷石膏进行电子背散射衍射（EBSD）分析,根据被检样品衬度的区别探明磷石膏的杂质物相,利用X射线能谱分析（EDS）成分确定杂质物相组成;利用X射线光电子能谱（XPS）对硫酸钙晶体表面以及混合杂质物相作进一步分析。EBSD分析结果表明,磷石膏中杂质物相主要包括二氧化硅、氟硅酸钠、氟硅酸钾、氟磷酸钙、氟化钙、硫酸钡、硫化铁、三氧化二铝等,此外还有硅、铝、磷、氟等杂质混合组成的复盐物相,其中二氧化硅、硫酸钡、硫化铁、氟磷酸钙和三氧化二铝为独立赋存物相,氟硅酸钠和氟硅酸钾的物相则混合分布在硫酸钙晶体之间,氟化钙杂质与硅、铝、磷、氟杂质复盐物相结合赋存。XPS分析结果表明,磷石膏中还存在硅酸钙、氟化铝、氟化镁、硫酸铝、磷酸铝、磷酸钙、磷酸氢钙和磷酸二氢钙等物相,其中磷酸钙、磷酸氢钙、磷酸二氢钙和氟磷酸钙四种物相的特征峰位分布极为接近。采用EBSD-XPS组合分析方法,不仅确定了磷石膏中杂质的物相,还阐明了杂质物相与硫酸钙晶体之间的构效关系。该研究为磷石膏杂质物相分析提供新途径,为磷石膏除杂净化及其综合利用提供坚实的理论依据。