On the theory of the alignment of a polar molecule in a non-polar solvent

The alignment average of a dilute polar substance dissolved in a non-polar solvent may be derived from the NMR spectrum of the polar component. Likewise, the Kerr effect is related to this quantity.In this paper a series expansion of the alignment of a polar molecule in a non-polar solvent is presented for a rigid-lattice model, first introduced by Van Vleck for the dielectric constant of a very dilute solution. Different results up to second order in the dipolar interaction are obtained for a simple, a body centred, or a face centred cubic lattice and a lattice whose lattice points are uniformly distributed in a continuum. The latter formula is compared with that calculated on the basis of the Lorentz local field model and with that of Buckingham in which the interaction between the dipoles themselves is neglected.     

Dielectric friction on a polar molecule rotating in a fluid

We consider the continuum model for dielectric friction on a polar molecule rotating in a fluid. Taking account of electrohydrodynamic coupling we calculate the frequency dependent rotatory friction coefficient to second order in the strength of the permanent dipole moment.     

Dielectric relaxation of polar molecules in mixtures of non-polar solvents.

The relaxation times of some rigid polar molecules are studied in very diluted mixtures of heptane-carbontetrachloride and heptanedecaline, Exponential relation between relaxation time and a characteristic length of the polar solute (r_eff) is found. The structure factor of the mixture which contains the contributions of both solvents cannot be composed by adding the features of each component only. Furthermore, the measured collision times seem not to follow a simple sum rule.     

Collective alignment of polar filaments by molecular motors

We study the alignment of polar biofilaments, such as microtubules and actin, subject to the action of multiple molecular motors attached simultaneously to more than one filament. Focusing on a paradigm model of only two filaments interacting with multiple motors, we were able to investigate in detail the alignment dynamics. While almost no alignment occurs in the case of a single motor, the filaments become rapidly aligned due to the collective action of the motors. Our analysis shows that the alignment time is governed by the number of bound motors and the magnitude of the motors’ stepping fluctuations. We predict that the time scale of alignment is in the order of seconds, much faster than that reported for passive crosslink-induced bundling. In vitro experiments on the alignment of microtubules by multiple-motor covered beads are in qualitative agreement. We also discuss another mode of fast alignment of filaments, namely the cooperation between motors and passive crosslinks.     

Alignment of polarizable dipoles on a lattice

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.     

Dielectric constant of non-polar fluids

The theory of the static dielectric constant of non-polar fluids is re-examined using graph-theoretical techniques. A sample of arbitrary shape in an applied static electric field E ₀ is considered. The non-local tensor which transforms from E ₀ to the electrostatic polarization P is expressed as a sum of graphs. The relation appears simpler in terms of the inverse tensor, which is also obtained as a sum of graphs. Agreement with classical electrostatics is then manifest, and the shape-independence of the dielectric constant follows. An approximation suitable for numerical work is formulated, and a simplified version is solved analytically. The results are similar to those for fluids of rigid polar molecules in the Mean Spherical Model approximation. Reasons for both the similarities and the differences are given.     

Modified theory of photoinduced molecular polar alignment in azo polymers

An improvement on the simplified theory of all-optical poling [Chem. Phys. Lett. 286, 415 (1998)] is proposed. In this improvement the influence of the cis isomer is taken into account in the process of photoinduced molecular polar alignment. An analytical expression for the induced polar order in the steady-state regime is derived. This expression shows that, although the contribution to the photoinduced second-order susceptibility from the cis state itself can be neglected, the population of the cis state plays an important role. This result suggests the possibility that one can improve the induced polar order by increasing the poling temperature, so that thermal-assisted optical poling can be explained.     

Dielectric dispersion studies of aromatic ketones in non-polar solvents

Measurements of dielectric constant (ε′) and loss (ε″) have been made at five different microwave frequencies from 1000 MHz to 67.7 GHz for acetophenone, benzophenone and propiophenone in four different non-polar solvents namely benzene, cyclohexane, 1-4-dioxane and n-heptane. Results are reported at five different temperatures from 25°C to 60°C. The dielectric data of these ketones in different solvents are analysed in terms of the Cole-Cole arc plots and superposition of two Debye-type absorptions. Values of mean relaxation times (τ_o), dipolement (μ), overall relaxation time (τ₁) and group relaxation time (τ₂) have been obtained and presented here. The values of relaxation time and dipolemoment are in reasonable good agreement, at the temperatures, at which there are available known data.     

Calculation of the field of a lattice of point magnetic dipoles

The magnetic field H produced by a spatially periodic distribution of point magnetic dipoles is expanded in a Fourier series. Because of a rapid (exponential) falloff of its harmonic amplitudes, the field H at an arbitrary point is the sum of the fields of only several nearest layers (or chains) of dipoles. The method is applied to calculate the magnetic fields at interstitial sites of Sm in different antiferromagnetic states.     

Extremely small domain in the lattice of magnetic dipoles

Lattices of magnetic dipoles with 1–4 rows are investigated. Numerical analysis reveals the smallest stationary domains formed in the lattices, necessary conditions for the formation and destruction of such domains are obtained, and the change in the magnetic moment of the lattices during domain formation is considered. It is shown that the action of an external field on one of the dipoles forming a domain is sufficient for its breaking. The lattices in which the orientational phase transition appears upon perturbation of several dipoles and propagates over the entire system are revealed.     

Long-range orientational order in a two-dimensional degenerate system of dipoles on a square lattice

It is shown that in a planar degenerate system on a square lattice with dipole-dipole interactions of electric or magnetic moments the thermodynamic fluctuations destroy the Goldstone mode in the spectrum of orientational oscillations. This gives rise to layered antiferroelectric structures characterized by a nonzero long-range orientational order parameter, which in the low-temperature limit is proportional to exp(−T|lnT|). Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 623–627 (25 April 1996)     

激光场和温度对氢氰酸分子准直的影响

通过分裂算符方法求解氢氰酸(HCN)分子的核转动波包,研究了激光场脉冲宽度、载波频率、激光峰值强度以及转动温度等参数对HCN分子准直程度的影响.结果表明,在温度T=10 K,使用脉冲宽度为100 fs,波长为800 nm,激光峰值强度I4×10~(13)W/cm~2范围内的激光场准直HCN分子时,最高准直度随光强增加升高,并且随光强增加,最高准直度增速变缓.在温度T=10 K,准直激光脉冲宽度为100 fs,波长为800 nm,峰值强度I=2×10~(13)W/cm2时,随激光载波频率在0.38×10~(15)Hzv1.13×10~(15)Hz范围内增加,HCN分子最高准直度出现振荡,在载波频率v=(0.4+0.1·n)×10~(15)Hz(n=1,2,3)时,最高准直度取极大值.在准直激光脉冲宽度为100 fs,波长为800 nm,峰值强度I=2×10~(13)W/cm2时,随温度在0 KT90 K范围内上升,准直度明显下降.     

Profile imaging of reconstructed polar and non-polar surfaces of ZnO

The atomic scale surface structures of ZnO non-polar as well as and ±(0 0 0 1) polar surfaces have been directly imaged by high-resolution transmission electron microscopy (HRTEM). The observations were made on clean surfaces created by irradiating a single ZnO nanobelt using 400 keV electron beam in TEM, under which ZnO dots were grown epitaxially and in situ on the surface of the nanobelt. A technique is demonstrated for directly distinguishing the surface polarity of the ±(0 0 0 1) polar surfaces. For the non-polar surface, HRTEM images and simulation results indicate that the Zn ions in the first and second layer suffer from inward and outward relaxation, respectively; the oxygen ions in the first and second layer prefer shifting to vicinal Zn ions to shorten the bonding distance. For the oxygen-terminated polar surface, the oxygen ions at the outmost top layer were directly imaged. a × 2 reconstruction has also been observed at the surface, and its atomic structure has been proposed based on image simulation. Oxygen-terminated polar surface is flat and shows no detectable reconstruction. For the Zn-terminated (0 0 0 1) polar surface, HRTEM may indicate the existence of Zn vacancies and a possibly c-axis, random outward displacement of the top Zn ions. Our data tend to support the mechanism of removal of surface atoms for maintaining the stability of (0 0 0 1) polar surfaces.     

Effect of the lattice alignment on Bloch oscillations of a Bose-Einstein condensate in a square optical lattice

We consider a Bose-Einstein condensate of ultracold atoms loaded into a square optical lattice and subject to a static force. For vanishing atom-atom interactions the atoms perform periodic Bloch oscillations for arbitrary direction of the force. We study stability of these oscillations for non-vanishing interactions, which is shown to depend on an alignment of the force vector with respect to the lattice crystallographic axes. If the force is aligned along any of the axes, the mean field approach can be used to identify the stability conditions. On the contrary, for a misaligned force one has to employ the microscopic approach, which predicts periodic modulation of Bloch oscillations in the limit of a large forcing.     

Classical theory of the molecule alignment in a laser field

In the classical mechanics framework the solution is given for the problem of the two-atomic molecule alignment in a field of linearly polarised laser pulse. The numerical results are presented for the Cl₂ molecule, together with the analytical results in the limit of a very short and strong laser pulse. The width of the angular distribution for molecular axes in field is demonstrated to depend on the radiation intensity as S ^?1/4, the alignment along the polarisation direction is possible as well as in the transversal direction. The analysis of some experimental data is given.     

Initial growth processes of Ag on polar and non-polar semiconductor substrates

Initial growth processes of Ag on both InSb(111)A and α-Sn(111) substrates at room temperature have been investigated by using reflection high-energy electron diffraction and Auger electron spectroscopy. The results show that the growth features are quite different for each system: Ag grows in the Stranski-Krastanov mode on InSb(111)A, while for Ag/α-Sn(111) the majority of the deposited Ag atoms are consumed in forming an Ag-Sn alloy. Discrete variational-Xα calculations showed that an onset of such growth modes is closely related to interfacial chemical bonding features for both systems.     

Charge exchange of a polar molecule at its cation

The Landau-Herring method is used to derive an analytic expression for the one-electron exchange interaction of a polar molecule with its positively charged ion, induced by a σ-electron. Analogously to the classical Van der Pole method, the exchange interaction potential is averaged over the rotational states of colliding particles. The resonant charge-transfer cross section is calculated, and the effect of the dipole moments of the core on the cross section is analyzed. It is shown that allowance for the dependence of the exchange potential on the orientation of the dipole moments relative to the molecular axis may change the dependence of the cross section on the velocity of colliding particles, which is typical of the resonant charge exchange, from the resonance to the quasi-resonance dependence.