平行板间方势阱偶极流体的结构性质研究

在密度泛函理论(DFT)框架下, 应用改进的基本度量理论(MFMT)表达硬球作用对自由能泛函的贡献, 根据统计力学理论结合加权密度近似(WDA)表达偶极作用对自由能泛函的贡献,得到了方势阱偶极流体在平行板间的密度分布表达式, 计算了偶极流体在两平行板间的密度分布, 并探讨了方势阱深度和宽度对体系密度分布的影响. 此外, 通过体系密度分布, 进一步分析了方势阱宽度和深度以及板间尺度与溶剂化力的关系.     

平行板间方势阱偶极流体的结构性质研究

在密度泛函理论(DFT)框架下, 应用改进的基本度量理论(MFMT)表达硬球作用对自由能泛函的贡献, 根据统计力学理论结合加权密度近似(WDA)表达偶极作用对自由能泛函的贡献,得到了方势阱偶极流体在平行板间的密度分布表达式, 计算了偶极流体在两平行板间的密度分布, 并探讨了方势阱深度和宽度对体系密度分布的影响. 此外, 通过体系密度分布, 进一步分析了方势阱宽度和深度以及板间尺度与溶剂化力的关系.     

基于电子共振跃迁的光频本征型超常电磁介质

超常电磁介质因其异于普通材料的电磁性质而受到广泛关注,基于周期性谐振结构的超常介质(即超材料)到光频段以后会出现加工困难,且损耗明显,而基于材料本征性质的超常介质却表现出独特的优势.文章简要介绍了三类本征型超常电磁介质的形成机制,它们分别是强各向异性机制、自旋波和等离子体耦合机制以及电子共振跃迁机制.重点介绍了基于电子共振跃迁的超常介质.采用半经典理论分析了气态介质中二能级、三能级体系的电偶极跃迁和磁偶极跃迁过程,总结了极化率和磁化率的一般表达式,综述了基于二能级、三能级及多能级模型获得的气态超常电磁介质.此外,对凝聚态体系中的电子共振跃迁和超常电磁性质关系作了简要阐述.最后,对采用实验方法实现的这种本征型超常电磁介质进行了简要分析.     

亲水性两性离子聚合物刷的构象与结构

本文应用分子场理论,研究暴露于水蒸气中的亲水性两性离子聚合物(HP)刷的构象与结构.理论模型考虑HP-水(P-W)氢键和水-水(W-W)氢键效应,以及HP单体之间的偶极-偶极相互作用.研究发现,P-W与W-W氢键决定着HP的水合性,P-W氢键形成,会诱导HP刷溶胀.我们通过考察HP单体间的偶极-偶极相互作用发现,随着偶极-偶极相互作用增强,HP链在垂直培基表面沿着链方向,形成了结节状结构.这是由于HP单体之间的偶极-偶极静电吸引作用导致单体间汇聚结节,这种结节在刷内产生了较强的排斥体积作用,因此,这种HP刷具有抗污性能.在较高的接枝密度环境下,由于HP链间单体之间的偶极-偶极静电吸引作用,会形成链间单体-单体的结节,在刷内形成结节网络状凝胶结构,这种结构的出现,会使得HP刷呈现极强的抗污性.另外,当体系中水蒸气浓度增加、水合相互作用增强时,增加的P-W氢键将平衡HP单体之间的偶极-偶极相互作用,使得结节解开,聚合物链伸展.我们的理论结果符合实验观测,由此表明,P-W氢键效应,以及HP单体之间的偶极-偶极相互作用决定着HP刷的构象转变和结构特性,刷内出现的两性离子聚合物链内单体间的结节和链间单体结节状凝胶结构,是两性离子聚合物刷呈现较强抗污性的本质特性.     

原子偶极压缩的相干控制和Cauchy-Schwarz不等式的破坏

通过研究孤立二能级原子与双模纠缠相干光场的相互作用,分析了体系中原子偶极压缩量子效应的时间演化规律.体系中作用场模的性质决定了原子偶极压缩程度;同时定义了体系的Cauchy-Schwarz不等式破坏参数ΔV,研究了不同条件下参数ΔV的时间演化特性,Cauchy-Schwarz不等式破坏程度和体系中所具有的非经典特性是一致的;即可以通过调控相干场参数来远程控制体系中的非经典特性. 关键词： 量子光学 双模纠缠相干态 偶极压缩效应 Cauchy-Schwarz 不等式     

纠缠态原子偶极间相互作用对量子态保真度的影响

研究了一对具有偶极偶极相互作用的纠缠态原子与相干态光场的相互作用,考察了原子间偶极偶极相互作用对体系中量子态保真度的影响.     

稠密共振介质中近偶极-偶极相互作用的局域场效应

 为研究存在由近偶极-偶极相互作用诱导的局域场效应时超短强激光脉冲与稠密共振介质相互作用的特性,采用光与物质相互作用的半经典理论,建立了稠密二能级体系中考虑原子间近偶极-偶极相互作用的修正光学Bloch方程,并用四阶Runge-Kutta法数值求解了该方程。研究结果表明:局域场效应对稠密二能级体系中Bloch矢量的瞬态相干过程和稳态性质都具有强烈的调制作用。并由此提出了调控稳态粒子数布居的两种方案。     

双模压缩真空场与耦合双原子相互作用系统中光场的量子特性

研究了双模压缩真空场与耦合双原子系统中光场的压缩性质和相干性质.讨论了光场的初始压缩因子、原子-场耦合常数及原子间偶极-偶极相互作用常数对光场量子特性的影响. 关键词： 双模压缩真空场 耦合双原子系统 光场的非经典性质     

同心椭圆柱-纳米管结构的双重Fano共振研究

提出了一种同心椭圆柱-纳米管复合结构,该结构由金纳米管中内嵌椭圆形金柱构成,利用时域有限差分法分析了尺寸参数、周围环境及纳米管内核材料对该结构光学性质的影响.结果表明,调节椭圆柱芯的旋转角度可产生双重偶极-偶极Fano共振,其主要是由椭圆柱芯的纵向或横向偶极共振模式与纳米管的偶极成键和反成键模式杂化形成的超辐射成键模式和亚辐射成键模式之间的相互作用产生的,且共振特性可通过调节复合结构的尺寸参数控制,随椭圆柱长轴或短轴的增大而红移,随纳米管外径的增大或整体尺寸的减小而蓝移,当纳米管内径增大时高频Fano共振随着红移,而低频Fano共振先蓝移再红移,同时其对外界环境的变化不敏感,但对纳米管内核材料变化有着较好的响应.利用等离激元杂化理论对该现象进行了解释.这些结果可为构造其他类型的多波段Fano共振二维或三维纳米结构提供一种新的方式.     

氖原子光电子角分布的理论计算

本文利用密度矩阵理论和Racah代数推导出了光电子角分布的一般计算公式, 并在多组态Dirac-Fock方法基础上发展了计算原子光电离过程中产生的光电子角分布的相对论程序, 利用该程序对氖原子2s和2p光电子角分布的偶极和非偶极参数进行了具体计算, 所得结果与已有文献具有很好的一致性. 在此基础上, 本文讨论了光子与电子相互作用多级展开中的非偶极项以及入射光的极化性质对光电子角分布的影响.     

Weakly anisotropic and string fluid phases in magnetorheological systems

Strongly coupled magnetorheological (MR) systems differ from regular simple or complex fluids since the effective interparticle interaction is controlled by external fields and thus provides a tunable anisotropic contribution. This, in turn, causes remarkable diversified phase diagrams of MR-systems. Our analysis of fluid-fluid transitions is based on a modified Ornstein-Zernike (OZ) equation which allows us to calculate structural properties of magnetorheological fluids with anisotropic (dipolar) interactions. The results are compared with MC simulations.     

From realistic to simple models of fluids. III. Primitive models of carbon dioxide,hydrogen sulphide and acetone,and their properties

Recently developed methodology to construct primitive models of associating fluids as direct descendants of complex realistic intermolecular potential functions [L. Vl?ek, I. Nezbeda. Molec. Phys., 102, 485 (2004).] is extended to polar fluids and applied to three substances of practical importance: quadrupolar carbon dioxide, and dipolar hydrogen sulphide and acetone. It is shown that the structural properties (in terms of the site–site correlation functions) of the primitive models of polar fluids reproduce very well those of their parent realistic models but, nonetheless, they perform worse than in the case of associating fluids. A number of thermodynamic properties of the developed models obtained by computer simulations is also reported (for their later use in theoretical investigation) and discussed.     

An equation of state contribution for dipolar and quadrupolar square-well fluids

A dipolar–quadrupolar contribution to the residual Helmholtz energy for a polar square well (a square well plus either a point dipole or a point quadrupole) fluid is developed based on the Padé approximation. Taking the square well system as reference, the contribution is formulated using an expansion for radial distribution function of the reference system. In addition to square well potential parameters the contribution depends only on dipole and quadrupole moments. This term is added as perturbation to a generalized equation of state for square well fluids. The results are then compared with the available simulation data in the literature. With the new equation obtained, it was possible to predict liquid–vapour equilibrium properties and critical properties of polar square well fluids more accurately than with available perturbation theories for multipolar square well systems. Application of the equation of state to a real dipolar (water) and a real quadrupolar (carbon dioxide) fluid indicated that the polar contribution greatly improved the predictions of saturation properties. Accurate prediction of critical properties for polar square well fluids remains as a challenge. This work can be useful in the development of better equations of state.     

Effect of short- and long-range forces on the properties of fluids. III. Dipolar and quadrupolar fluids

Using realistic pair potential models for acetone and carbon dioxide, both the spatial and orientational structure of these two typical multipolar (i.e. dipolar and quadrupolar, respectively) fluids is investigated in detail by computing the complete set of the site-site correlation functions, multipole-multipole correlation functions, and selected 2D correlation functions. The effect of the range of interactions on both the structural and thermodynamic properties of these fluids is studied by decomposing the potential into short- and long-range parts in the same manner as for water [Kolafa, J. and Nezbeda, I., 2000, Molec. Phys., 98, 1505; Nezbeda, I. and Lísal, M., 2001, Molec. Phys., 99, 291]. It is found that the spatial arrangement of the molecules is only marginally affected by the long-range forces. The effect of the electrostatic interactions is significant at short separations and cannot be neglected but nevertheless the overall structure of the short-range and full systems is similar as well as their dielectric constants. These findings are also reflected in the dependence of the thermodynamic properties on the potential range with the short-range models providing a very good approximation to those of the full system.     

Effect of an electric field on the surface tension of a dipolar-quadrupolar fluid and its implication for sign preference in droplet nucleation

The effect of a uniform electric field on interfacial properties of dipolar-quadrupolar fluids is investigated by using the density-functional theory. As in the case of purely dipolar fluids the (thermodynamic) surface tension is always altered by the external field, regardless of the direction of the field. However, unlike the purely dipolar fluids, for two given external fields with the same strength but exactly opposite direction the magnitude of variation in the surface tension is different. This apparent symmetry breaking by reversing the field direction suggests a new molecular mechanism to explain the phenomenon of sign preference in droplet formation on charged condensation centers.     

Towards a unified view of fluids

It has traditionally been believed that, unlike normal fluids whose structural properties are determined primarily by the intermolecular short-range repulsive interactions, the properties of polar and associating fluids are strongly affected by the long-range Coulombic interactions. In the course of investigations to determine the primary driving forces governing the behaviour of various (non-simple) fluids, and hence to gain a deeper understanding of the molecular mechanisms leading to the development of theoretically based simple models and theory, extensive and systematic computer simulations have been performed on typical quadrupolar (carbon dioxide), dipolar (acetone and acetonitrile), and associating (hydrogen fluoride, methanol, and water) fluids using the available realistic effective pair potentials and their variants involving forces of different ranges. In addition to the main structural characteristics (one- and two-dimensional site–site correlation functions, local g factors, and radial slices through the full pair correlation function), the dielectric constants and the thermodynamic properties (internal energy and pressure) of both the homogeneous liquid and supercritical fluid phases, and vapor–liquid equilibria have also been considered. Furthermore, in the case of water, the diffusion coefficient and viscosity have also been considered along with water at the interface. All the obtained results lead to the unambiguous conclusion that the structure, defined in terms of the complete set of site–site correlation functions, for both polar and associating pure fluids is governed by the same molecular mechanism as for normal fluids, i.e. by the short-range interactions (which, however, may be both repulsive and attractive), whereas the long-range part of the electrostatic forces, regardless of their strength, plays only a marginal role and may be treated as a perturbation only. The consequences of these findings for theory and applications are also discussed.     

Accurate measurement of H(N)-H(alpha) residual dipolar couplings in proteins.

A method for accurately measuring H(N)-H(alpha) residual dipolar couplings is described. Using this technique, both the sign and magnitude of the coupling can be determined easily. Residual dipolar coupling between H(N)(i)-H(alpha)(i) and H(N)(i)-H(alpha)(i-1) were measured for the FK506 binding protein complexed to FK506. The experimental values were in excellent agreement with predictions based on an X-ray crystal structure of the protein/ligand complex, suggesting that these residual dipolar couplings will provide accurate structural constraints for the refinement of protein structures determined by NMR.     

Accurate Measurement of H–H Residual Dipolar Couplings in Proteins

A method for accurately measuring H^N–H^α residual dipolar couplings is described. Using this technique, both the sign and magnitude of the coupling can be determined easily. Residual dipolar coupling between H^N(i)–H^α(i) and H^N(i)–H^α(i-1) were measured for the FK506 binding protein complexed to FK506. The experimental values were in excellent agreement with predictions based on an X-ray crystal structure of the protein/ligand complex, suggesting that these residual dipolar couplings will provide accurate structural constraints for the refinement of protein structures determined by NMR.     

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.     

Long-lived states in solution NMR: theoretical examples in three- and four-spin systems

Long-lived spin states have been observed in a variety of systems. Although the dynamics underlying the long lifetimes of these states are well understood in the case of two-spin systems, the corresponding dynamics in systems containing more spins appear to be more complex. Recently it has been shown that a selection rule for transitions mediated by intramolecular dipolar relaxation may play a role in determining the lifetimes of long-lived states in systems containing arbitrary numbers of spins. Here we present a theory of long-lived states in systems containing three and four spins and demonstrate how it can be used to identify states that have little or no intramolecular dipolar relaxation.