一种寻找团簇异构体的准动力学方法

建立了一套普遍适用的寻找团簇异构体的准动力学方法,该方法能够迅速给出在一般气相生长条件下形成概率较大的异构体.用该方法得到了C21的异构体谱,并采用分子动力学方法模拟了21个自由的碳原子在氦气氛中形成稳定团簇的过程,表明动力学过程中形成概率较大的异构体都已包含在该方法所得到的异构体谱中.所得到的C21最稳定结构的势能远低于采用遗传算法所得到的结果(Chem. Phys. Lett. 364 213,2002). 关键词： 团簇 异构体 最低势能 全局优化     

密度泛函理论对ZrnPd团簇结构和性质的研究

用B3LYP/LANL2DZ方法对Zr_nPd(n =1—13)团簇的平衡几何结构、能量、频率、电子性质和磁性进行了计算.研究表明,Pd原子位于表面的异构体更为稳定,其中Zr₇Pd,Zr₁₂Pd团簇稳定性高,是幻数团簇,此外,相对于Zr_nCo与Zr_nFe团簇,Zr_nPd团簇参与化学反应的能力较弱,化学稳定性更     

第一性原理计算ZrnFe(n=2—13)团簇的基态结构及其磁性

从第一性原理出发，利用密度泛函理论中的广义梯度近似对Zr_nFe(n=2—13)团簇进行了结构优化、能量和频率计算.在充分考虑自旋多重度的前提下，对每一具体尺寸的团簇，得到了多个平衡构型，并根据能量高低确定了团簇的基态结构.综合团簇的结合能、二阶能量差分以及团簇的最高占据轨道和最低未占据轨道间的能隙可知Zr₅Fe，Zr₇Fe和Zr₁₂Fe团簇的稳定性相对较高，Zr₁₂Fe团簇的结构是具有I_h对称性的正二十面体，而且Zr₁₂Fe的稳定性在所有团簇中是最高的.另外，不仅Zr₅Fe，Zr₇Fe和Zr₁₂Fe团簇的稳定性相对较高，而且它们均为磁性团簇(而Zr_n团簇的磁矩在n≥5时已经发生了淬灭)，由此可知通过选择合适的掺杂元素可能得到高稳定的磁性团簇.从Mulliken布居分析结果可知，除了在Zr₁₂Fe团簇中Fe原子失去少量电荷外，其他团簇中Fe原子均从Zr原子那里得到了一定量电荷，即Fe原子在Zr_nFe(n=2—13，n≠12)团簇中是电子受体.     

小型二氧化钛团簇(TiO2)n (n=2-8)的低能异构体研究

钛和氧之间存在多种成键方式,但迄今为止,二氧化钛团簇均只有少数几种异构体被报道. 与广泛使用的全局优化方法不同,本工作通过优化大量的随机初始结构,获得(TiO₂)_n (n=2-8)团簇稳定异构体. 首先利用PM6半经验方法对高达一万个以上的初始结构进行初步的优化筛选,并对筛选出的结构进行进一步的DFT计算以获得二氧化钛团簇的稳定异构体. 利用这种策略,发现了大量未经报道的稳定异构体,并提出了(TiO₂)₅和(TiO₂)₈新的最稳定异构体. 这些结构中包括含3个末端氧原子的异构体、含5配位氧原子和6配位钛原子的异构体等未经报道的新结构类型. 与丰富成键特征相对应,发现异构体数目随团簇尺寸的增大而急剧增加,对于(TiO₂)₇和(TiO₂)₈,能量在30 kcal/mol以内的异构体都在50个以上. 该工作发现了大量的二氧化钛小型团簇异构体,并凸显了其多样的结构特征,增进了对二氧化钛纳米团簇的结构、成键的理解,并为进一步的理论模拟、力场优化等提供了理论基础.     

团簇异构体生长概率的理论预测

由于实验表征团簇结构的困难,理论预测团簇在生长过程中各种异构体的生成概率便十分重要.考虑到分子动力学可模拟高温时各异构体的相对丰度,把问题归结为高温时所形成的各种异构体在室温环境中如何转化,并建立了统计力学模型,得到了异构体相互演化所需时间与环境温度的解析关系.以C₁₂团簇为例,在2500 K时所形成的最可几异构体在室温环境中需要超过10¹²年才能转化为最低势能异构体. 关键词： 团簇 异构体 统计物理 分子动力学     

NaBn(n=1—9)团簇的几何结构和电子性质

应用密度泛函理论(DFT)中B3LYP方法在6-311+G(d)水平上计算并分析了 NaB_n(n=1—9)团簇的几何结构及电子性质. 同时, 讨论了团簇的平均结合能、能级间隙、二阶能量差分和极化率.研究表明:NaB_n(n=1—9) 团簇基态绝大多数为平面构型. 能级间隙和二阶能量差分结果表明NaB₃与NaB₅是幻数团簇. 另外, 对平均线性极化率和极化率的各向异性不变量研究表明基态NaB_n团簇的电子结构随B原子的增加虽然趋于紧凑, 但尚未形成特定的堆积方式.     

ConCm±(n=1-5; m=1,2)团簇的密度泛函理论研究

本文采用基于自旋极化的密度泛函理论系统研究了Co_nC_m^± (n=1-5; m=1,2)团簇的几何结构和电子结构特性. 将Co_nC^± (n=2-5)团簇中的一个Co替换为C原子, 个体的基态几何结构发生明显变化; 在Co_nC₂^± (n=1-5)团簇的生长序列中, 发现从n=3开始团簇中的两个C原子有彼此分离分布的趋势, 我们分析, 这是Co金属能够维持单壁碳纳米管(SCNTs)保持开口生长, 成为非常有效的一种催化剂的重要原因. 同时, 将Co_nC^± (n=2-5)团簇中添加一个Co原子后系统的总磁矩出现大幅下降的趋势, 但仍保持奇偶交替的规律. 通过比较中性及带电的Co_nC以及Co_nC₂ (n=1-5)团簇的碎裂能, 本工作发现: 由实验获取的SCNTs应均为带正电的体系, 这一结论与已有的实验模型拟合得很好.     

铁基纳米晶合金的介观阻抗率及应用

提出了铁基纳米晶合金介观阻抗率的物理概念，用Maxwell方程组求得其计算公式ρ=-μ{A}/{t}/Δ×H，该式表明铁基纳米晶合金的介观阻抗率与材料内部的介观磁场强度H、介观磁矢势A和介观磁导率μ有关，磁矢势A是由介观结构引起的量子力学效应，是由合金的微观结构决定的.这个理论很好地解释了铁基纳米晶粉末、粉芯     

AunCo (n=1～7)团簇的密度泛函理论研究.

用B3LYP/LanL2DZ方法,系统研究了钴掺杂金团簇Au_nCo (n=1～7)可能的稳定结构和最低能量异构体的相对稳定性．确定了系列低能量异构体,部分异构体的电子态具有较高的自旋多重性．计算结果表明,除n=7外Au_nCo (n=1～7)团簇的基态几何结构为平面结构,Au₂Co为幻数团簇、具有较高的稳定性.     

铋镓共掺的高浓度掺铒石英基光纤中铒离子团簇率的实验研究

使用MCVD法结合溶液掺杂技术制作了铒铋镓共掺的石英基光纤.根据光纤吸收谱，得出其中铒离子浓度是5.24×10²⁵/m³.然后提出了一种新的利用透射率测定掺铒光纤的团簇率的实验方法，该方法相比于以前的方法更简单有效.利用该方法，测得了自制铒铋镓共掺光纤中团簇率.通过与前人的结果相比较，充分说明铋和镓的掺入大大提高了铒离子在石英基光纤中的溶解度，抑制了铒离子形成团簇.     

Microscopic basis for Fick's law for self-diffusion

We investigate self-diffusion in a classical fluid composed of two species which are distinguished through the color of their particles, either black or white, but are identical as regards their mechanical properties. Disregarding color the fluid is in thermal equilibrium. We show that if a single test particle in the one-component fluid moves asymptotically as Brownian motion, then the color density and current in certain classes of nonequilibrium states are related, on the appropriate macroscopic scale, through Fick's law, and the former is governed by the diffusion equation. If in addition several test particles move asymptotically as independent Brownian motions, then the colored fluid is, on a macroscopic scale, in local equilibrium with parameters governed by the solution of the diffusion equation.Part of this work was done while both authors were at IHES, Bures-sur-Yvette, France.Supported in part by NSF Grant No. PHY 78-15920-02.Supported by a Heisenberg fellowship of the Deutsche Forschungsgemeinschaft.     

Expressions for third-order aberration theory for holographic images

Expressions for third-order aberration in the reconstructed wave front of point objects are established by Meier. But Smith, Neil Mohon, Sweatt independently reported that their results differ from that of Meier. We found that coefficients for spherical aberration, astigmatism, tally with Meier’s while coefficients for distortion and coma differ.     

Convolution identities for quasiprobabilities for Bose functions

We present identities relating the equations of motion of various quasiprobabilities for quantum oscillators. These identities turn out useful for checking the consistency of approximations made in constructing the equations of motion with the basic Bose commutator. Moreover, our identities allow to identify the quasiprobability distributions which have the easiest-to-solve equations of motion.     

Requirements for Optical Systems for Virtual Displays

Currently, there are many types of virtual displays and new types arise every year. However, their vast majority appear technically inapplicable because they are inconsistent with a number of key factors related to the eye physiology. In this paper, we present the general requirements for optical systems of virtual displays, taking into account human eye physiology.     

Analytical relations for luminescence intensities accounting for reabsorption

Mathematical relations defining distortions from true values of photoluminescence intensities that arise during measurements due to absorption at the luminescence frequencies are derived for various experimental designs. The adequacy of the derived relations is confirmed by special experiments. The relations allow one to define correctly the contours, widths, and maximum frequencies of luminescence bands of absorbing media and to deduce absorption coefficients of substances from luminescence measurements. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 788–795, November–December, 2008.     

Figure of merit for spectrometers for EDXRF

How should one select the best detector for a particular measurement in energy dispersive X‐ray fluorescence (EDXRF)? How should one select the optimum system configuration, i.e. the best shaping time and beam current? Manufacturers provide a variety of specifications, such as energy resolution and maximum count rate, but these are indirectly related to the end use of an EDXRF instrument, the measurement and detection limit of the measured elemental concentrations. We suggest in this paper using the time required to achieve a given statistical uncertainty as a figure of merit. We derive scaling rules for this figure of merit based on conventional specifications, including energy resolution, peaking time, maximum count rate, detector area, and intrinsic efficiency. These scaling rules also include the peak to background ratio of a photopeak and the number of overlapping peaks. We then show how this figure of merit can be used to select the optimum detector and spectrometer configuration for specific applications and compare the results to data obtained with typical systems. Copyright © 2012 John Wiley & Sons, Ltd.     

内窥镜校正中棋盘格标志点自动提取方法

为了解决具有大畸变内窥镜标定的标志点提取困难和人工参与较多等问题,提出了一种简单快速准确的棋盘格标志点提取方法。该方法是采用粗提取和优化来完成棋盘格标志点的提取的:首先利用高斯滤波去除图像噪声,并计算两次图像的梯度特征,利用梯度极值约束得到粗提取的角点;然后利用棋盘格标志点的对称特点消除噪声点的影响,并得到优化的角点位置。实验结果表明,该方法简单易行,计算量小,不需要人工干预即可获得全部标志点,反投影误差小于0.2pixel。     

Memory for pitch versus memory for loudness

The decays of pitch traces and loudness traces in short-term auditory memory were compared in forced-choice discrimination experiments. The two stimuli presented on each trial were separated by a variable delay (D); they consisted of pure tones, series of resolved harmonics, or series of unresolved harmonics mixed with lowpass noise. A roving procedure was employed in order to minimize the influence of context coding. During an initial phase of each experiment, frequency and intensity discrimination thresholds [P(C) = 0.80] were measured with an adaptive staircase method while D was fixed at 0.5 s. The corresponding physical differences (in cents or dB) were then constantly presented at four values of D: 0.5, 2, 5, and 10 s. In the case of intensity discrimination, performance (d') markedly decreased when D increased from 0.5 to 2 s, but was not further reduced when D was longer. In the case of frequency discrimination, the decline of performance as a function of D was significantly less abrupt. This divergence suggests that pitch and loudness are processed in separate modules of auditory memory.     

Map for Simultaneous Measurements for a Quantum Logic

In this paper we will study a function of simultaneous measurements for quantum events (s-map) which will be compared with the conditional states on an orthomodular lattice as a basic structure for quantum logic. We will show the connection between s-map and a conditional state. On the basis of the Rényi approach to the conditioning, conditional states, and the independence of events with respect to a state are discussed. Observe that their relation of independence of events is not more symmetric contrary to the standard probabilistic case. Some illustrative examples are included.