The holographic investigation of diffusion of azo-dye in liquid crystals host

The diffusion of azo-dye (DR-1) in a planar liquid crystals host (5CB) at various temperatures has been investigated by laser-induced holographic grating relaxation technique. The decay of the diffraction intensity provides information about the diffusion of photoexcited azo-dye molecules. The relaxation time constants can be derived from the time dependence of the diffraction intensity fitted by a single exponential function. Thus, the diffusion coefficients parallel and perpendicular to the director of liquid crystals at various temperatures can be obtained from the plots of the reciprocal of the relaxation time versus the square of the grating vector. From the analysis of the holographic grating relaxation, the diffusion is faster along the molecular director than for the perpendicular case, and the diffusion increases with rising temperature either parallel or perpendicular to the nematic director of liquid crystals. PACS 42.40.Eq; 42.40.Lx     

Diffusion relaxation of photoinduced gratings in polyvinyl acetate latex films

The features of the postexposure relaxation of holographic gratings recorded in inhomogeneous polyvinyl acetate latex films with photosensitive agents (photochromic molecules of fulgide dyes and phenanthrenequinone) have been considered. The diffusion coefficients and rms displacements of izomerized probe in polymer latex particles and aqueous environment are determined within the model of two diffusion states. The effective diffusion coefficient of the molecular probe, which is responsible for the relaxation of gratings, increases with an increase in their period in wet films, whereas in dry films, this parameter is independent of the grating period. In the films subjected to high-temperature treatment the effective diffusion coefficient decreases with an increase in the grating period. The successive stages of grating relaxation in latex films with phenanthrenequinone are related to the diffusion of free molecules, radicals, and polymer chains, as well as to the local displacement of macromolecular segments at distances of 5–25 nm.     

不同极性溶液中MEH-PPV链内激子扩散的研究

本文采用飞秒时间分辨瞬态光栅技术研究了MEH-PPV在不同极性溶液中的内激子扩散行为. 采用宽带连续白光同时探测瞬态吸收和瞬态光栅信息. 系统地研究了MEH-PPV的振动失相行为以及单激子弛豫和粒子数弛豫动力学. 结果表明,在极性较大的溶剂中,样品溶液的弛豫扩散过程会加快.     

A combination of laser-induced grating and transient-absorption experiments for investigation of laser pulse properties and fast molecular relaxation processes

A repetitive low-power laser-pulse apparatus has been developped which allows both absorption relaxation and light-induced grating experiments without changing geometry or components. The influence of pulse width and coherence time on the diffracted intensity correlation function is discussed for a weak amplitude grating in terms of a simplified theory. From the corresponding absorption relaxation signals including the coherent coupling contribution some easy ways for detecting vibronic-relaxation, intersystem-crossing and orientational-relaxation times are deduced. The advantage of the in-situ measurement of the amplitude grating autocorrelation function leads to a precise zero-delay calibration of the transient absorption equipment. Furthermore one gets the response function for the absorption experiments from the grating experiments, if stable mode-locking operation of the argon laser is reached. This condition can be controlled either by the time course of the absorption or grating signals. A surprisingly short coherence length is detected for the cavity-dumped laser beam. A reliable check of the coherent-coupling theory confirms the theoretical assumptions incorporated into the fast-relaxation signal analysis. Experiments on dye molecules show high triplet yield of heavy-atom substituted dyes and fast rotational diffusion of oblong molecular rotors.     

Decay of excitonic gratings: Effect of exciton diffusion and reabsorption of exciton fluorescence

A phenomenological theory of the decay time of an excitonic grating (a grating of exciton densities) formed at some initial moment in a plane-parallel plate of a molecular crystal has been developed. Along with the diffusion of excitons reabsorption of the fluorescence light radiated by excitons is also taken into account. It is shown that in crystals with large overlapping of absorption and emission spectra the life time of the exciton grating is diminished appreciably by reabsorption. Numerical estimations for this life for anthracene at room temperature were made. In the above conditions it is the reabsorption which is important, whereas the diffusion of excitons hardly affects the decay time of the excitonic grating. The available experimental data and possible trends of further investigations are discussed.     

Lineshapes of the 172 and 602 GHz rotational transitions of HCN

Experimental lineshapes of the 172 and 602 GHz millimeter lines of HC¹⁵N in collision with H₂, N₂, O₂, CH₃CN and some rare gases are studied for the purpose of atmospheric applications and detailed collision analysis. Using of a sensitive frequency modulation technique allows highlighting clear deviations from the usual Voigt profile, these departures being generally considered as related to molecular diffusion effects or to the dependence of collisional relaxation rates on molecular speeds. Except the light buffer gases H₂ and He, the linefits using the Galatry profile lead to nonlinear pressure dependencies of relaxation rates, that rules out the frequent hypothesis of an exclusive role of molecular diffusion (Dicke effect). This observation is shown to be in accordance with the fact that optical radii related to relaxation are usually greater than kinetic Lennard-Jones radii tied to diffusion. In contrast, the actual lineshapes are well reproduced by the Speed-Dependent Voigt profile taking into account the speed dependence of relaxation rates which display linear pressure dependencies. For the particular cases of the N₂- and Ar-HCN pairs, the experimental results are rather well explained by semi-classical computations based on the Robert–Bonamy formalism improved by the model of exact trajectories. These computations show that relaxation rates are proportional to some power of the colliders’ relative speed. A detailed comparison of relaxation parameters deduced from low-pressure experiments with Galatry and Speed-Dependent Voigt profiles allows to infer that the optical diffusion rates are much smaller that kinetic ones, so that the experimental lineshapes depend nearly exclusively on the speed dependence of relaxation rates and are weakly affected by molecular diffusion effects. Extension of these conclusions to other molecules of atmospheric interest is discussed. Finally, an appendix presents unpublished results dealing with the collisional relaxation of some rotational lines of HC¹⁵N (at 258 GHz for different temperatures and at 344 GHz) and HC¹⁴N (at 355 GHz).     

Determination of the rotational diffusion tensor of macromolecules in solution from nmr relaxation data with a combination of exact and approximate methods--application to the determination of interdomain orientation in multidomain proteins

In this paper we present a method for determining the rotational diffusion tensor from NMR relaxation data using a combination of approximate and exact methods. The approximate method, which is computationally less intensive, computes values of the principal components of the diffusion tensor and estimates the Euler angles, which relate the principal axis frame of the diffusion tensor to the molecular frame. The approximate values of the principal components are then used as starting points for an exact calculation by a downhill simplex search for the principal components of the tensor over a grid of the space of Euler angles relating the diffusion tensor frame to the molecular frame. The search space of Euler angles is restricted using the tensor orientations calculated using the approximate method. The utility of this approach is demonstrated using both simulated and experimental relaxation data. A quality factor that determines the extent of the agreement between the measured and predicted relaxation data is provided. This approach is then used to estimate the relative orientation of SH3 and SH2 domains in the SH(32) dual-domain construct of Abelson kinase complexed with a consolidated ligand.     

Optical measurement and control of spin diffusion in n-doped GaAs quantum wells

Transient spin gratings are used to study spin diffusion in lightly n-doped GaAs quantum wells at low temperatures. The spin grating is shown to form in the excess electrons from doping, providing spin relaxation and transport properties of the carriers most relevant to spintronic applications. We demonstrate that spin diffusion of the these carriers is accelerated by increasing the density or energy of the optically excited carriers. These results can be used to better understand and even control spin transport in experiments that optically excite spin-polarized carriers.     

液晶中激光感生热光栅的研究

本文中探讨了MBBA液晶中,由热致位相光栅产生的简并四波混频效应。通过测量热光栅衰变的弛豫时间,得到T=45.1℃时,液晶各向同性相的热弥散系数为D=8.6×10^{-4)cm2·S-1)。 关键词：}     

Relationship between structural and stress relaxation in a block-copolymer melt

The relationship between structural relaxation on molecular length scales and macroscopic stress relaxation was explored in a disordered block-copolymer melt. Experiments show that the structural relaxation time, measured by x-ray photon correlation spectroscopy is larger than the terminal stress relaxation time, measured by rheology, by factors as large as 100. We demonstrate that the structural relaxation data are dominated by the diffusion of intact micelles while the stress relaxation data are dominated by contributions due to disordered concentration fluctuations.     

Theory of laser-induced amplitude and phase gratings including photoselection,orientational relaxation and population kinetics

The generation of amplitude and phase gratings in an absorbing medium is discussed, including the effects of light polarization, population kinetics and orientational relaxation. A theory is given which describes light diffraction by a combination of an amplitude and a phase grating. It is shown that the coherent coupling effect in transient absorption measurements is solely due to the amplitude grating. Weak absorption and a low triplet yield favour diffraction of the probing beam by the phase grating in triple-beam experiments. Phase grating experiments can be performed with highly soluble absorber dyes, the absorption maximum of which is far away from the laser wavelength. Due to photoselection, diffraction by an amplitude grating depends on the polarization of the probing beam while phase grating diffraction is insensitive to this. Perpendicular polarization of the two exciting pulses prevents the buildup of a phase grating and results in light diffracted by the amplitude grating that is proportional to the transient dichroism signal as known from the common absorption relaxation.     

Theoretical description of the methyl group relaxation in liquids

Spin-lattice and spin-spin relaxation times of protons and deuterons of methyl groups in liquids are calculated using a semiclassical form of the density operator formalism and symmetry adapted spherical tensors. The model of molecular motion assumes diffusion of the whole molecule containing the methyl group with some internal rotation. Special consideration is given to uniaxial internal rotation and anisotropic diffusion as well as to biaxial internal rotation and isotropic diffusion in interpreting the experimental results.     

瞬态自旋光栅系统的建设及其在自旋输运研究中的应用

简要介绍了瞬态光栅系统原理及光路的建设,包括瞬态光栅的产生与探测.采用了外差探测法（heterodyne detection）,大大提高了信噪比.利用瞬态自旋光栅系统,研究了(110)方向生长的本征GaAs/AlGaAs单量子阱中自旋输运特性,测得室温下电子自旋的扩散常数D_s=551 cm/s. 关键词： 瞬态自旋光栅 自旋扩散 自旋输运 自旋弛豫     

Theory of intramolecular spin relaxation by translational diffusion in locally ordered fluids

In locally ordered fluids, such as macromolecular solutions, clays and lyotropic liquid crystals, nuclear spin relaxation can be induced by modulation, through translational diffusion of the fluid molecules, of the magnitude and orientation of the residual intramolecular spin-lattice coupling tensor, which is only partially averaged by local molecular motions near an interface. A theory of spin relaxation in locally ordered fluids bounded by planar interfaces is developed, with special emphasis on effects of translational diffusion. The theory is based on a continuous diffusion model (CDM) which, in contrast to the commonly adopted discrete exchange model (DEM), treats equilibrium and time-dependent distribution functions in a self-consistent way. A striking feature of translational diffusion in heterogeneous systems is the abundance of reencounters with previously visited interfacial regions. It is demonstrated that these diffusional reencounters, which are inherent in the CDM theory, may lead to a relaxation behaviour which is qualitatively different from that predicted by the DEM theory. Furthermore, it is seen that the widespread concept of intrinsic relaxation rate (associated with a spatial region) and the fast/slow exchange classification are not generally valid. The formal framework of the CDM theory allows molecular interactions of any complexity to be introduced. In this paper a mean-field model based on the nonlinear Poisson-Boltzmann equation is used to obtain analytic expressions for the spectral density functions that determine the relaxation behaviour in the presence and in the absence of spectral line splittings.     

Collisional Relaxation of Luminescence Anisotropy in Rarefied Gases in the Condensed Phase

The orientational relaxation of optically induced anisotropy in rarefied gases and at a damped rotation has been investigated. It has been found that the anisotropy relaxation in rarefied gases is described by a reduced kinetic equation depending only on free rotation integrals. The behavior of the integral anisotropy of luminescence for free symmetric and asymmetric top molecules has been elucidated. The law of luminescence depolarization has been obtained for asymmetric top molecules in the Gordon J-diffusion model. It represents the sum of two Stern–Volmer-type dependences, whose relative contribution is determined by the orientation of the dipole moments of transitions with absorption and emission of light in the molecular coordinate system and by the principal moments of inertia of the molecular top. It has been established that in the limit of a strongly damped rotation, kinetic equations of the general form reduce to equations of rotational diffusion. A number of modified diffusion equations correctly describing the contribution of inertial effects to the orientational relaxation of anisotropy have been obtained.     

Low-frequency proton NMR relaxometry of a polymer dispersed liquid crystal above TNI

Using proton NMR relaxometry in the kilohertz frequency range, we study dynamics of 5CB liquid crystal molecules dispersed in the form of spherical microdroplets in a PDLC material. The focus of the study is the spin-lattice relaxation in the rotating frame, T1rho(-1), measured above the nematic-isotropic transition TNI. We show that the relaxation rate T1rho(-1)--when induced by uniform molecular translational diffusion in a spherical cavity--depends on the strength of the rotating magnetic field as T1rho(-1) proportional to omega1(-alpha) where alpha varies between 0.7 and 1, depending on the thickness of the ordered surface layer. This relaxation mechanism governs mainly the transverse spin relaxation, whereas the measurements of the frequency and temperature dependence of T1rho(-1) indicate a strong effect of slowing-down of molecular translational diffusion in contact with the polymer surface and yield the average dwell-time of molecules at the surface of the order 10(-5) s.     

Investigation of the long-term stability of arc-induced gratings heat treated at high temperatures

A long-period grating written in the SMF-28 fibre was heat treated at 1000 °C for 15 days. The spectrum of the grating shifted to longer wavelengths and the resonances depth decreased as a result of structural relaxation. The background loss increased considerably for times longer than 200 h, and this loss is caused by devitrification of the fibre.     

Detection of monolayer gratings of adsorbates by linear optical diffractions

We report the detection of linear optical diffraction of a He-Ne laser probe beam from a monolayer grating of molecular adsorbates up to the fifth order with no electronic or electromagnetic enhancement. The advantages of using linear diffraction to probe surface diffusion of adsorbates are discussed.     

Measurement of the diffusion-length of carriers and excitons in CdS using laser-induced transient gratings

Using a laser-induced grating arrangement we measure the probe beam diffraction intensity under stationary excitation conditions as function of the grating constant. We developed a mathematical model and extract by comparison with experimental data the diffusion lengths of the quasiparticles existing at various temperatures and excitation levels in the CdS sample from the experimental findings.     

Proton magnetic relaxation studies of molecular diffusion in the liquid crystal 40.5

Nuclear magnetic relaxation spectroscopy is used to study the molecular dynamics in a liquid crystal butoxy benzylidene pentylaniline (40.5) in the frequency range 4 to 30 MHz and the results are compared with two other members of the same homologous series (viz 40.8 and 40.6). Spin lattice relaxation time studies indicate that molecular self diffusion (SD) and reorientation processes (R) dominate the relaxation process and their relative contributions are quantified. This contrasts with the case where order director fluctuations (ODF) effectively mediate relaxation process and all the three processes are found to be important in 40.6 in a similar frequency range.T _1D in 40.5 in the nematic phase shows temperature dependence indicating that ODF that is present at low frequencies might be diffusion assisted. These relaxation data are analysed in theS _B phase of this compound also to obtain contributions to the relaxation process. These results are also analysed to obtain different parameters associated with the above dynamical processes.