晶体X射线衍射的发现及其深远影响

1912年4月,弗里德里希、克里平和劳厄成功地观察到X射线透过硫酸铜晶体后的衍射斑点!随后劳厄推导出描述晶体衍射的著名劳厄方程.由于晶体X射线衍射的发现,劳厄于1914年荣获诺贝尔物理学奖.1912年10月,W.L.布拉格通过X射线透射ZnS晶体实验,推导出了著名的布拉格方程.1915年布拉格父子荣获诺贝尔物理学奖.晶体X射线衍射的发现对自然科学的影响是深远的.2012年是劳厄发现晶体X射线衍射100年,文章回忆了这段光辉的历史及其对科学技术所产生的深远影响,以怀念科学先驱们对科学技术的贡献,弘扬他们对科学研究的认真严谨的科学态度、勇于创新的科学精神和谦逊无私的品德.     

X射线晶体学的起源

1912年是值得纪念的一年.在这一年里,先是劳厄(M.Laue,1879—1960)等发现了X射线的晶体衍射现象并给出了一个解释,此后不久布拉格(W.L.Bragg,1890—1971,后简称小布拉格)又作了重新解释并借此获得了有关晶体结构的信息,从而开始了X射线晶体学发展艰难但成果辉煌的历史.作为整个X射线晶体学史研究的第一步,本文先对其起源作一全面的历史考查. 一、X射线晶体衍射现象 的发现和劳厄的解释 发现X射线晶体衍射现象的直接起因还得先从厄瓦耳说起.1912年初,他正在慕尼黑大学著名理论物理学家索末菲教授的指导下撰写《各向同性共振子的各向异性…     

从劳厄发现晶体X射线衍射谈起

文章从劳厄发现晶体X射线衍射的前因后果谈起．劳厄的这个发现产生了两个新学科，即X射线谱学和X射线晶体学．文中还回顾了布拉格父子对这两个新学科所作的重大贡献，并阐述了X射线晶体学的深远影响．     

纪念劳厄发现晶体X射线衍射90周年编者的话

20 0 2年和 2 0 0 3年分别是劳厄 (vonLaueMax)发现X射线晶体衍射和布拉格 (BaggWL)建立布拉格方程90周年 .劳厄和布拉格的发现是 2 0世纪物理学意义深远的大事 .1895年伦琴发现X射线后 ,关于X射线的本质是不清楚的 ,一种观点认为是穿透性很强的中性微粒 (粒子学说 ) ,另一种观点认为是波长较短的电磁波 (波动学说 ) .应该说 ,劳厄的发现 ,除了他本人具备坚实的物理基础 ,敏锐的洞察能力以及当时劳厄所在的慕尼黑大学高水平的学术研究环境等因素外 ,还直接得益于与埃瓦尔德 (EwaldPP)的一次谈话 .通过与埃瓦尔德的讨论 ,劳厄酝酿出一个…     

纪念劳厄发现晶体X射线衍射100周年

文章从劳厄发现晶体X射线衍射的前因后果谈起.劳厄的这个发现产生了两个新学科,即X射线晶体学和X射线光谱学.文章中还回顾了布拉格父子对开展这两个新学科研究所作出的卓越贡献,并略述了X射线晶体学所产生的深远影响.     

小布拉格（Bragg，Sir William Lawrence，1890-1971）英国物理学家（Physicist，England）

他在测定晶体结构的基础上，提出X射线衍射的布拉格公式，因此他和父亲共获1915年诺贝尔物理学奖。他生于澳大利亚，在阿德莱德大学求学，1909年到英国剑桥大学三一学院学物理学、1912年他在劳厄发现X射线通过晶体产生衍射的基础上，进行了一系列实验。1913年发表布拉格公式，这是用X射线衍射分析晶体结构的基本公式，他还用他父亲设计的X射线分光计精确测出X射线的波长。     

晶体的X射线衍射理论--劳厄与埃瓦尔德的遗产

文章概述了晶体X射线衍射理论的发展过程，介绍了不同形式的衍射几何理论、劳厄方程、布拉格方程与埃瓦尔德作图法，讨论了它们之间的异同之处．文章还介绍了运动学衍射理论和不同形式的动力学衍射理论，后者包括达尔文的简化处理、埃瓦尔德的表述和劳厄的表述，并对其物理后果进行了讨论，强调了动力学理论所预言的电磁波在周期结构中传播所引起的能隙．     

声学劳厄衍射和描述劳厄衍射斑的晶格模型

波在晶格中的衍射是波动学和固体物理学领域中的重要课题。本文第一部分描述了一个与在晶格中X射线的劳厄衍射相似的声学实验;第二部分主要讨论描述立方晶体中劳厄衍射斑的晶格模型。     

纪念劳厄发现X射线衍射100周年

100年前,德国人劳厄发现X射线通过晶体时可以发生衍射效应．随后,英国的布拉格父子等人发展出一系列实验和分析方法,利用X射线晶体衍射解析出具有原子分辨率的分子结构．在过去的100年中,X射线衍射分析对世界的科学发展乃至人们的生活都产生了至关重要的影响,并且在这100年间,X射线衍射实验方法和分析方法也有了长足进步．硬X射线自由电子激光的出现为X射线衍射分析进一步发展提供了更广阔的空间,可以预期,基于自由电子激光的X射线衍射分析会进一步在物理、化学、生物等学科中发挥更为重要的影响．     

纪念劳厄发现射线衍射周年

100年前,德国人劳厄发现X射线通过晶体时可以发生衍射效应.随后,英国的布拉格父子等人发展出一系列实验和分析方法,利用X射线晶体衍射解析出具有原子分辨率的分子结构.在过去的100年中,X射线衍射分析对世界的科学发展乃至人们的生活都产生了至关重要的影响,并且在这100年间,X射线衍射实验方法和分析方法也有了长足进步.硬X射线自由电子激光的出现为X射线衍射分析进一步发展提供了更广阔的空间,可以预期,基于自由电子激光的X射线衍射分析会进一步在物理、化学、生物等学科中发挥更为重要的影响.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.