Effect of symmetry lowering on the band structure of polyacetylene

In an attempt to apply band-structure calculations to polymers with “reduced” symmetries, and simultaneously to examine some of the causes of band gaps in semiconducting polymers, two types of symmetry reduction are applied on the all-trans polyacetylene chain. Using a neighboring-segment approximation the consequent perturbations were used to calculate changes to the band structure of the undistorted chain and the energy gaps compared with those obtained from a direct calculation on the distorted polymer.     

The perturbation of a polymer molecular orbital

The symmetry restrictions imposed by the Born-Kármán boundary conditions on a polymer in order to calculate its electronic band structure are relaxed by the separate introduction of two types of perturbation at a site on the polymer chain. The first simulates a defect or the close approach of a molecule by modifying the segment orbital functions in the environment of the site while the second simulates a change in the distance between one pair of adjacent segments by altering the interaction between them. When applied to a molecular orbital described by a Bloch function and arbitrary energy band these perturbations generate a small number of electronic states whose energies lie above and below those of the band. The sign of the energy shift depends on the symmetry of the crystal orbital perturbed.     

The configuration of diphenyl in the crystalline and in the vapour states: A simple non-bonded H-H potential function

Electron diffraction shows that in the vapour the phenyl rings of diphenyl are inclined at about 42°. This is a compromise between the P _z-orbital overlap, which tends to keep the whole molecule planar, and overcrowding of the 22 and 66 hydrogen atoms which causes twisting of the rings to reduce the steric repulsion. A potential function, having an exponential form, has been derived, which gives a minimum at the observed angle.

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Zusammenfassung Die Elektronenbeugung zeigt, daß die beiden Ringe des Diphenyls im Gaszustand um einen Winkel von etwa 42° gegeneinander verdreht sind. Dieser Zustand stellt einen Kompromiß dar zwischen der Überlappung der p _z-Orbitale einerseits, die das ganze Molekül planar zu halten sucht, und der Pressung der van der Waals-Radien der 2,2 und 6,6-Wasserstoffatome andererseits, die die Ringe aus der gemeinsamen Ebene herausdreht, um die sterische Hinderung abzumindern. Für die potentielle Energie wurde eine e-Funktion mit einem Minimum am beobachteten Winkel hergeleitet.

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Résumé La diffraction électronique indique qu'en vapeur les anneaux phényles du diphényle s'inclinent à 42° approximativement. Cet angle est un compromis entre le recouvrement des orbitales P _zqui tend à maintenir plan la molécule entière, et l'encombrance des atomes d'hydrogène 22 et 66 qui font tordre les anneaux pour réduire la répulsion stérique. On a dérivé une fonction potentielle, d'une forme exponentielle, qui a un minimum à l'angle observé.

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The authors acknowledge the award of a maintenance grant to one of us (D.A.M.-B.) by H.M. Department of Scientific and Industria Research.     

螺旋桨空化噪声连续谱仿真算法

螺旋桨空化噪声连续谱由大量空泡随机崩溃所辐射噪声叠加而成。据此,考虑到空泡初始崩溃时刻的随机性和各空泡辐射噪声频谱的相似性,对四种情况空化辐射噪声连续谱特性分别进行了理论分析表明,可依据空泡半径和初始崩溃时间的统计性质,采用蒙特卡罗方法对空化噪声连续谱波形进行仿真建模,模型参数为空泡半径期望值、差异度和随机度,分别相应于确定辐射噪声的基准波形、各空泡噪声频谱相似程度和大量空泡的随机崩溃过程。该方法输入参数物理意义明确,控制简单,能较好地仿真空化噪声连续谱的特性,仿真结果与理论分析吻合较好。     

新的基于条纹投影轮廓测量的系统标定方法

提出一种新的光栅条纹投影轮廓测量术系统标定模型,新模型不要求投影装置和成像系统的光心连线与参考面平行、成像系统的光轴垂直于参考面及投影装置和成像系统的光轴相交。基于该模型得出了新的相位高度映射关系,其待定系数与成像点的坐标无关。实际测量中只需2个高度不同的标准块便可以求得待定系数。对4个标准块进行高度测量,得到的最大相对误差为0.6%。实验证明:该标定方法简单有效,提高了系统标定的可操作性和测量精度。     

(113)面硅衬底上自组织生长的GeSi量子点及其光荧光

用透射电子显微镜观察了Si(113)衬底上由固态源分子束外延生长的自组织量子点的形貌,测量了其原生及退火后低温下的光荧光谱.对所得结果进行了分析. 关键词：     

大离轴量凹面取样镜的设计

根据系统的使用要求以及椭球面特有的成像性质,提出使用离轴椭球面来完成光束取样和完善成像的设计方式.利用设计输入,计算出镜面的初始结构参数,得到离轴椭球面的二次曲线方程,将计算结果带入光学设计软件ZEMAX中验证成像质量.结果表明:离轴椭球面不但将物点和像点分开,减小了系统的长度,而且在像点处成完善像,将离轴成像转换为轴上成像,使后续光斑成像系统的高质量成像易于实现.     

基于彩色光栅投影的快速三维测量方法

 针对条纹投影术提取物体高度信息的问题,提出了一种新的基于彩色光栅投影的三维测量方法。对选取G分量为255,R,B分量各取0或255而组成的青、白、黄、绿四色遵循格雷码原理进行编码,然后将G分量作正弦调制形成投影光栅投向被测物体。提取采集到的光栅变形图中G分量,利用傅里叶变换方法得到其初始相位;同时对采集到图像中的R,B分量作阈值迭代分割而G分量自动赋值为255,综合三分量信息得到条纹颜色信息进而获取条纹的周期信息,从而展开相位。全过程仅需投影一幅彩色光栅图就能完成三维测量,实验结果表明该算法易于实现,在测量实时性和精确性上表现良好。