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1.
The anchoring of nematic liquid crystals on surfaces with grafted liquid-crystalline chain molecules is studied by computer
simulations and within a mean-field approach. The computer simulations show that a swollen layer of collectively tilted chains
may induce untilted homeotropic (perpendicular) alignment in the nematic fluid. The results can be understood within a simple
theoretical model. The anchoring on a layer of mutually attractive chains is determined by the structure of the interface
between the layer of chain molecules and the solvent. It is controlled by an interplay between the attractive chain interactions,
the translational entropy of the solvent and its elasticity. A second-order anchoring transition driven by the grafting density
from tilted-to-homeotropic alignment is predicted.
Received 5 July 2001 and Received in final form 16 November 2001 相似文献
2.
V.S.U. Fazio L. Komitov C. Radüge S.T. Lagerwall H. Motschmann 《The European physical journal. E, Soft matter》2001,5(3):309-315
The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers
was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the
orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary
flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the
flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the
nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by
the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic
orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced
quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state
the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a
slight preference for the direction of the previous flow.
Received 8 November 2000 and Received in final form 12 March 2001 相似文献
3.
4.
The dynamic alignment and post-ionization alignment of nitrogen molecules are investigated while considering the effect of tunneling ionization. The effects of tunneling ionization on the angular distribution are calculated when the molecules are irradiated by different laser intensities. The results show that laser intensity directly affects the time and extent of dynamic alignment. Furthermore, the extent of post-ionization alignment is not only determined by laser intensity but also affected by the final extent of dynamic alignment. The post-ionization alignment will dominate during the process of molecular (or molecular ion) rotational alignment for femtosecond laser pulse. The time of tunneling ionization is a significant factor to the final ensemble angular distribution of molecular ions when laser intensity is low. 相似文献
5.
We present a method to discuss simultaneously the relative importance of molecular dynamic and geometric alignment induced by intense laser fields in theoretical view. This method divides the process of molecular alignment into three steps, which are tightly correlated with that of molecular multielectron dissociative ionization and Coulomb explosion. A fourth-order Runge-Kutta algorithm and a developed counting approach are used to calculate the angular distribution of molecules in the first and second steps of molecular alignment. The last step is described by a field-ionization, Coulomb explosion model. The angular distribution of molecules at the critical distance originated from geometric alignment is obtained by calculating the volume of shells associated with a series of particular angle. The final angular distributions of molecules are obtained by properly weighting the results of three steps. The numerical results of distinguishing between dynamic and geometric alignment for certain conditions are presented and discussed. Our computational results show that the alignment mechanism, which dominates the observed anisotropy of angular distributions of ionic fragments for a given condition, is determined by the dependences of the extent of dynamic and geometric alignment on laser parameters and molecular parameters. 相似文献
6.
Rui-Han Zhu Chun-Cheng Wang Si-Zuo Luo Xue Yang Mei-Xia Zhang Fu-Chun Liu Da-Jun Ding 《Frontiers of Physics》2013,8(2):236-240
Nonadiabatic alignment by intense nonresonant laser fields is a versatile technique to manipulate the spatial direction of molecules. By solving the time-dependent Schrödinger equation numerically the degree of alignment of the molecules initially in different rotational state are calculated and the results show that the degree of alignment strongly depends on the initial rotational state. Thus, the present study indicates that, for obtaining a high degree of alignment for molecules, appropriate selection of molecular rotational states is necessary. 相似文献
7.
Guiping Zeng Fengjiao Zhong Chengyin Wu Hongbing Jiang Qihuang Gong 《Laser Physics》2009,19(8):1691-1696
Field-free molecular alignment can be achieved by nonadiabatic rotational excitation of molecules with strong femtosecond laser pulses. We experimentally and theoretically demonstrate that the degree of alignment can be improved with multi-pulse excitation. An approach is proposed to tune the frequency of few-cycle pulses using field-free alignment of molecules. 相似文献
8.
Dependences of dynamic alignment of CO molecules induced by intense femtosecond laser fields on laser wavelength, intensity and pulse duration are investigated by numerical simulations. A counting approach and a fourth-order Runge-Kutta algorithm are used to calculate the angular distribution and the time evolution of molecules. A two-step Coulomb explosion model of diatomic molecules in intense laser fields is used to determine the instant that CO molecular dynamic alignment is over. Our calculating results show that the linear polarizability and the damping force play an important role in the angular rotation of CO molecule in conditions of 800 nm laser wavelength and 1015 W/cm2 laser intensity. The contributions of the second-order field-induced dipole moment and the higher-order correction term to molecular rotation acceleration comparing to the linear polarizability and damping force are negligible. The extent of dynamic alignment of CO molecules reduces with the increasing of laser intensity. The dynamic alignment time of CO molecules is tightly connected to the laser pulse duration. The angular distributions of CO molecules as the laser pulse length varied from 50 to 250 fs at laser intensity of 3×1014 W/cm2 are shown and discussed. 相似文献
9.
Kinetic energy release of diatomic and linear triatomic molecules in intense femtosecond laser fields
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The kinetic energy release of fragment ions produced by the interaction of femtosecond laser pulse radiation with diatomic and linear triatomic molecules N_2, CO, CO_2 and CS_2 is investigated. In the case of linear polarization, angles at which the kinetic energy release of ions has the maximum value are different from the alignment of molecules though the kinetic energy release of fragment atomic ions depends on the angle between the laser polarization vector and the detection axis of the time-of-flight. For the diatomic molecules, the critical internuclear distance in multielectron dissociative ionization with a circularly polarized light is larger than that with a linearly polarized light. For linear triatomic molecules, our data indicate that a concerted Coulomb explosion process is a universal phenomenon in the interaction of molecules with intense laser fields, even in the circularly polarized regime. During two C-O (or C-S) bonds breaking simultaneously, the C ion obtained larger energy in circular polarization than that in the linear polarization. Different variations of kinetic energy release between the diatomic and the linear triatomic molecules are discussed. 相似文献
10.
Xiaoxia Zhong Xiuqin Yu Qu Li Shouyu Luo Yingli Chen Yu Sui Jie Yin 《Optics Communications》2001,190(1-6):333-337
The alignment of azobenzene molecules in DR19-PUI film induced by all-optical poling is identified. When the writing beams of frequencies ω and 2ω are both linearly polarized with their polarization directions parallel to each other, azobenzene molecules tend to reorient to perpendicular direction (i.e. direction perpendicular to the polarization of writing beams). At the end of the writing process more molecules orient in perpendicular direction than those in parallel direction (i.e. direction parallel to the polarization of writing beams). The alignment of molecules in parallel or perpendicular direction is, respectively, characteristic of noncentrosymmetry or centrosymmetry. It is the alignment of molecules in parallel direction that results in the second-order nonlinearity in the poled film. 相似文献
11.
The alignment average of polar molecules may be derived from their NMR spectrum, when a strong electric field is applied to the liquid. The Kerr effect can likewise be related to the alignment.In this paper the alignment has been calculated for a rigid-lattice model, previously developed by Van Vleck in the theory of dielectric polarization and extended by others. A series expansion of the alignment for a system of isotropically polarizable dipoles is presented up to second-order terms in the dipolar interaction. The result for a continuum has been compared with those calculated on the basis of the Lorentz and Onsager model of the liquid.An explicit expression for the total electrostatic energy of a binary mixture of polar and non- polar molecules with a symmetric polarizability tensor has also been calculated; it has been obtained from a generalization of a formalism, developed by Mandel and Mazur for the pure polar liquid. 相似文献
12.
Nonadiabatic alignment of rotationally cold N2 molecules is optimally controlled by shaping femtosecond pump pulses with the feedback of degree of alignment evaluated by an ion imaging technique. The alignment is optimized by doubly peaked pulses with approximately equal intensities. A doubly peaked pulse with an appropriate interval can be regarded as a single pulse with a center trough based on the considerations from both time and frequency domains, suggesting that the effective duration of a doubly peaked pulse rather than its structure is crucial to nonadiabatic molecular alignment. 相似文献
13.
O. V. Valba S. K. Nechaev M. V. Tamm 《Journal of Experimental and Theoretical Physics》2012,114(2):354-364
A new statistical approach to the problem of pairwise alignment of RNA sequences is proposed. The problem is analyzed for
a pair of interacting polymers forming an RNA-like hierarchical cloverleaf structures. An alignment is characterized by the
numbers of matches, mismatches, and gaps. A weight function is assigned to each alignment; this function is interpreted as
a free energy taking into account both direct monomer-monomer interactions and a combinatorial contribution due to formation
of various cloverleaf secondary structures. The binding free energy is determined for a pair of RNA molecules. Statistical
properties are discussed, including fluctuations of the binding energy between a pair of RNA molecules and loop length distribution
in a complex. Based on an analysis of the free energy per nucleotide pair complexes of random RNAs as a function of the number
of nucleotide types c, a hypothesis is put forward about the exclusivity of the alphabet c = 4 used by nature. 相似文献
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15.
A model of the alignment of azobenzene molecules in polymer film induced by all-optical poling is proposed and verified by experiment. We found that when the writing beams of frequencies ω and 2ω are both linearly polarized with their polarization directions parallel to each other, azobenzene molecules tend to reorient to the direction perpendicular to the writing beams polarization. At the end of the writing process, more molecules orient to the direction perpendicular to the writing beams polarization than those which orient to the parallel direction. The alignment of molecules parallel or perpendicular to the polarization of the writing beams is characteristic of polarity or no polarity, respectively. The alignment of molecules along the polarization of writing beams results in the second order nonlinearity in the polymer film. According to the model, a new method to improve the optical poling efficiency is put forward. 相似文献
16.
Florian Latteyer Sabine Savu Heiko Peisert Thomas Chass 《Journal of Raman spectroscopy : JRS》2012,43(9):1227-1236
This work presents an investigation of films prepared by doctor blade casting, the formation of self‐assembled microstructures of a liquid crystalline phthalocyanine with highly oriented molecules. Raman Spectroscopy in combination with atomic force microscopy is applied to study the structures within the films. By keeping the substrate at room temperature or at 353 K during coating, different geometric structures namely rods and islands form. Rod‐like structures are growing in coating direction, whereas directional growth of the islands is not observed. The distribution of the rod lengths varies widely, whereas the width appears more uniform. Annealing of the samples shows a different behavior of the two textures. Islands tend to melt, and rods smooth their structural form, which is extracted from Raman imaging in combination with atomic force microscopy. Additionally, Raman imaging gives insight into laterally different relative crystallinity. These observations are discussed in the context of the molecular orientation as probed by polarized Raman spectroscopy. These polarized Raman spectra indicate azimuthal alignment of the molecules within the rods (edge on alignment). This alignment occurs along and also perpendicular to the growth direction. In contrast to the alignment in the rods, the molecules inside the islands occurring at higher temperature do not show preferential molecular orientation. After annealing, no preferential molecular orientation is observed in rods because of the loss of anisotropy, too. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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18.
The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment. With the help of simulations, the degree of alignment is extracted from the data. 相似文献
19.
A technique was provided to nonintrusively quantify the field-free alignment of molecules by measuring the transient birefringence
caused by the aligned molecules. Pure heterodyne alignment signals were obtained from the difference of the signals under
an external electric field with opposite polarity and equal magnitude. The results demonstrated that the pure heterodyne alignment
signal directly reproduced the revival structure of molecular alignment and its Fourier transform spectroscopy represented
the actual populations of the different J states in the rotational wave packet. 相似文献
20.
本文利用三维经典系综模型研究了低强度周期量级脉冲驱动排列分子的非次序双电离. 结果表明, 电子对的关联特性强烈地依赖于分子的排列方向和激光脉冲的载波包络相位; 垂直分子反关联电子对的比例总是高于平行分子反关联电子对的比例; 当载波包络相位从0到π 逐渐增加时, 反关联电子对的数目先增加再减少; 对于平行分子, 电子对的释放总是以正关联为主; 而垂直分子的主导关联模式则依赖于激光脉冲的载波包络相位, 当载波包络相位为0.3π-0.7π之间时, 电子对以反关联释放为主, 其他相位下以正关联为主. 本文利用分子势能曲线和电子返回能量很好地解释了电子关联特性对分子排列方向和载波包络相位的依赖关系. 相似文献