气相中W~+活化CO_2分解的自旋禁阻反应机理

用密度泛函理论中的UB3LYP方法,对W采用相对论校正赝势基组(SDD),对C、O采用6-311+G(3d)基组,研究了气相中不同自旋态W+活化CO2分解的反应机理.计算结果表明,W+活化CO2分解反应以六重态进入反应通道,经过六重态势能面到四重态势能面的系间窜越(ISC),最后产物WO+和CO以四重态离开反应通道.运用Harvey方法优化出最低能量交叉点(MECP),并计算了MECP处的自旋-轨道耦合(SOC)常数(494.95cm-1),势能面的交叉和在MECP处较强的自旋-轨道耦合作用降低了自旋禁阻反应能垒,为反应提供了一条低能反应路径,反应总放热量为122.33kJ.mol-1.     

甲烷氧化偶联锂/氧化镁催化剂和纳米催化剂

采用初湿浸渍和溶胶凝胶法分别制备了Li/MgO催化剂和Li/MgO纳米催化剂. 比较两种Li/MgO催化剂对于甲烷氧化偶联反应的催化性能. 采用X射线衍射、BET吸附和透射电镜进行了表征.在973-1073 K和总压力为101 kPa下对催化剂进行了测试. 实验结果表明,Li/MgO纳米催化剂比普通催化剂对于甲烷氧化偶联反应表现为更高的甲烷转换率,较高选择性和较高的的主要产品(乙烷和乙烯)的产率.     

A completely iterative method for the infinite domain electrostatic problem with nonlinear dielectric media

We present an iterative method for the solution of the exterior all-space electrostatic problem for nonlinear dielectric media. The electric potential is specified on interior boundaries and the electric field decays at infinity. Our approach uses a natural variational formulation based on the total energy of the nonlinear dielectric medium subject to boundary conditions. The problem is decomposed into an exterior calculation and an interior calculation with the boundary-specified electric potentials imposed as constraints between them. Together, these enable an iterative method that is based on the variational formulation. In contrast to direct solution of the electrostatic problems, we avoid the construction, storage and solution of dense and large linear systems. This provides important advantages for multiphysics problems that couple the linear electrostatic Poisson problem to nonlinear physics: the latter necessarily involves iterative approaches, and our approach replaces a large number of direct solves for the electrostatics with an iterative algorithm that can be coupled to the iterations of the nonlinear problem. We present examples applying the method to inhomogeneous, anisotropic nonlinear dielectrics. A key advantage of our variational formulation is that we require only the free-space, isotropic, homogeneous Greens function for all these settings.     

Fourier transform microwave spectroscopy of LiCCH, NaCCH, and KCCH: Quadrupole hyperfine interactions in alkali monoacetylides

The alkali metal monoacetylides LiCCH, NaCCH, and KCCH and their deuterium isotopologues have been investigated using Fourier transform microwave (FTMW) spectroscopy in the frequency range 5-37 GHz. The molecules were synthesized in a supersonic expansion by the reaction of metal vapor, produced by laser ablation, with acetylene or DCCD. Use of target rods of the pure metal and a DC discharge immediately following the laser interaction region were significant factors in molecule production. Multiple rotational transitions were recorded for all species, except where only the J = 1 → 0 line was accessible (Li species). Quadrupole hyperfine interactions arising from the metal nuclei were resolved in each molecule, as well as those from the deuterium nucleus in the deuterated isotopologues. From a combined analysis with previous millimeter-wave data, refined rotational constants were determined for these species, as well as ⁷Li, ²³Na, ³⁹K, and D eQq parameters. The values of the metal quadrupole coupling constants are comparable to those of the alkali halides and hydroxides, indicating a similar degree of ionic character in the metal-ligand bond.     

Control of resonant peak depths of tunable long-period fiber gratings using overcoupling

We present a method to reduce changes in the resonant peak depth of a long-period fiber grating (LPFG) as the resonant band is tuned by varying the external refractive index. We theoretically analyze the effects of the initial coupling strength on the peak depth change as external refractive index is varied. By controlling the initial coupling strength, it is experimentally demonstrated that an optimum peak depth can be obtained over a range of operating wavelengths that will maximize the sensitivity and stability of LPFG based sensors and tunable filters.     

Emulator with inverse trend in first-order and second-order polarisation mode dispersion

We design a polarisation mode dispersion (PMD) emulator through subdividing a polarisation maintaining fibre (PMF) of 22 m in length. The aim of this emulator design is to show that first-order and second-order PMD can be inversely proportional to each other. Furthermore, the emulator is also used to show that the magnitude of PMD is independent to whether its statistics approach theoretical distributions or not, of most importance is the degree of mode coupling. The same (as former) applies to its autocorrelation function (ACF). The PMD control mechanism for the emulator is not in real time.     

Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

We report a simple fabricating approach to control the mode couplings in long-period fiber gratings (LPFGs) through side exposing fiber to long-pulse-10.6-μm laser from a cheap, internally modulated CO₂ tube. By tuning focused-spot size on fibers, not only circularly symmetric mode coupling but also asymmetric mode couplings can be effectively achieved. Simulation of mode profiles in grating cross-section with Finite Element Method (FEM), and LPFG-cladding etching experiment with hydrofluoric acid (HF), support our explanation that asymmetric mode coupling in LPFGs depends on local refractive-index (RI) change within an azimuthally thin cladding layer, resulted from large-spot method induced deep melt flow on fiber surface during CO₂ laser irradiation.     

混响室的概率统计分析方法及其蒙特卡罗模拟

 提出了一种研究混响室问题的理论分析方法,从理想混响室的离散概率模型出发,采用蒙特卡罗模拟来获得室内电磁场量及其相关物理量的概率统计特性。运用该方法获得了混响室内若干场量（均为归一化值）的概率密度函数、室内一段均匀无耗单导体传输线的负载平均耦合截面及负载电流响应模值的概率分布,所得结果与已有文献给出的结果（包括由解析公式计算或测量所得的结果）吻合良好。由于蒙特卡罗方法的普适性,采用该方法可以获得任意待分析参量的全部概率统计特性（只要样本数足够大）。同时该方法无需针对具体混响室进行,只要在其测试区域能较好地与理想混响室的基本概率模型相吻合,就能给出较精确的计算结果,且其计算量远远小于基于全波分析的蒙特卡罗方法。     

高速气流中激光加热平板数值模拟与分析

 采用流固耦合方法,数值模拟了高速流场中激光作用下来流速度对平板温度分布的影响。结果表明:无激光辐照时,高速气流中平板有较高的气动生热平衡温度,且平板-气流之间的换热系数随来流速度增大而增大;在平板前沿换热系数增长最快,沿平板长度方向增速趋于平缓。分析了激光辐照时高速气流中激光加热平板的温度分布情况,考察了来流速度不同时,气动生热、散热和激光辐照对平板温度的影响,给出了激光辐照后的温升情况和温度分布,分析了在不同速度来流下,对流散热、摩擦生热和激光加热之间的竞争关系,结果表明,平板温度具体分布主要是加热过程竞争的结果。     

Fast and accurate algorithm for the simulation of NMR spectra of large spin systems

The computational cost for the simulation of NMR spectra grows exponentially with the number of nuclei. Today, the memory available to store the Hamiltonian limits the size of the system that can be studied. Modern computers enable to tackle systems containing up to 13 spins [1], which obviously does not allow to study most molecules of interest in research. This issue can be addressed by identifying groups of spins or fragments that are not or only weakly interacting together, i.e., that only share weakly coupled spin pairs. Such a fragmentation is only permitted in the weak coupling regime, i.e., when the coupling interaction is weak compared to the difference in chemical shift of the coupled spins. Here, we propose a procedure that removes weak coupling interactions in order to split the spin system efficiently and to correct a posteriori for the effect of the neglected couplings. This approach yields accurate spectra when the adequate interactions are removed, i.e., between spins only involved in weak coupling interactions, but fails otherwise. As a result, the computational time for the simulation of 1D spectra grows linearly with the size of the spin system.