动态光散射实验

动态光散射技术是测量纳米及亚微米颗粒粒径的有效方法．本文介绍了动态光散射实验的原理、实验装置及方法，并测量了粒径为85nm的聚苯乙烯水溶液的粒径，分析了实验产生误差的原因．     

Dynamic light scattering in solid polymers

The viscosity of an amorphous polymeric solid above its glass transition [T _g (T,P)] increases as the temperature of the solid is decreased or the pressure is increased. Under changes in temperature or pressure, molecular subunits in the polymeric solid undergo configurational changes. Such changes or relaxations have a distribution of relaxation strengths and times. As the solid is cooled or as the hydrostatic pressure on the solid is increased, the relaxation strengths increase and the relaxation times increase. These changes in relaxation or dynamic properties are very dramatic as the empirical T _g is approached. Near T _g the polymeric solid is no longer in volume equilibrium; continued cooling or pressuring at a time rate faster than the average relaxation time will produce a polymeric glass. This glass is a nonequilibrium, amorphous solid. If the glass is held at a fixed temperature and pressure very close to, but below, T _g , the volume of the glass will be observed to relax to its equilibrium value. For temperatures and pressures well below T _g , equilibrium is a much more conjectural concept since the relaxation times become extremely long. It has been proposed^1,2 that there is a characteristic temperature T _g at which an amorphous polymer undergoes a second-order transition to an equilibrium glass with zero configurational entropy (i.e., a noncrystallizable solid).     

多模光纤式动态光散射实验研究

介绍了多模光纤在动态光散射中的应用,搭建了基于多模光纤的动态光散射实验系统,并用该系统测量了单分散、多分散以及不同浓度的标准聚苯乙烯乳胶球悬浮液.结果表明,该系统可准确地测量浓度达4.5%的聚苯乙烯乳胶球溶液中悬浮颗粒的粒径分布.     

Dynamic light scattering by charged silicon nanoparticles in colloid

The experimental results of diameter measurament silicon nanoparticles with using dynamic light scattering method are presented in this article. Colloids of nanosilicon in acetone, in methanol and in water were studied. The measurements were taken with nanotrak device (“Photocor complex,” dynamic light scattering instrument, produced by Photocor Instruments Inc.) during regular time intervals (about several minutes). The results from each solution were compared with the results in other solutions and the stable ones were examined. Results obtained in water appeared to be nonrecurrent. Electrical double layer of nanosilicon particles in liquid had been measured for all samples. It was established that the reason of such behavior of n-Si in water solution was double charge layer which changed hydrodynamical parameters of water.     

The spatial coherence factor in light scattering from a system of independent particles

We derive the spatial coherence factor associated with the intensity correlation function of light scattered from N independent particles. The expression agrees with that derived by assuming a gaussian light field. We show also that the so-called cross-spectral purity condition is valid in this case. The applicability of this expression to the case of clipped intensity correlation function and to the case of strongly interacting particles are also discussed.     

Magnetic-field-induced coherence in hyper-Rayleigh scattering

Second harmonic generation by a disordered array of superparamagnetic nanoparticles in a static magnetic field H is studied experimentally. For H = 0, only hyper-Rayleigh scattering takes place; i.e., the second-harmonic radiation is diffuse and unpolarized. For H ≠ 0, a coherent (specular and linearly polarized) component appears against the hyper-Rayleigh scattering background. The phase of the coherent component changes by π when the direction of the magnetic field is reversed. It is shown that the effect can be explained by correlations in the spatial fluctuations of the nonlinear-optical and magnetic polarizabilities of the particles, taking into account the nonuniformity of the effective (local) static magnetic field.     

Variable coherence scattering microscopy

We propose and demonstrate the feasibility of a new microscopic technique, which is based on variable coherence illumination. By manipulating the spatial coherence properties of an incident evanescent field, subwavelength resolution is achieved over a large field of view from far-field intensity measurements.     

Dynamic light scattering study of the interface evolution in porous media

The interface evolution during the evaporation of a liquid from a saturated layer of porous medium (paper) was experimentally studied using spectral analysis of intensity fluctuations of a laser radiation scattered by the layer. The data obtained were compared with the results of modeling the irreversible growth in three-dimensional lattices. The dependences of the spectral halfwidth of intensity fluctuations on the drying time demonstrate the characteristic features of drying front evolution, which proved to be similar to those found in the modeling of irreversible growth front. A comparison of the maximal halfwidths for two different saturating liquids suggests that the motion of local interfaces during the liquid evaporation from a layer of porous medium is close to the “classical” diffusion.     

Dynamic light scattering of a concentrated multicomponent spherical particle solution

The“measured”dunamic structure function of highly concentrated and smallspherical colloidal system with a narrow distribution of particle sizes can be expressed in termsof the sum of two independent modes due to collective diffusion and polydispersity fluctuationsrespectively.The“measured”static structure functions S~M(K) is derived,and applied to cal-culate the polydispersity distribution.The relationship between the moments of the gammaparticle size distribution and the moments determined by a dynamic light scattering measure-ment is developed for the case when the particles are small and spherical.     

Dynamic light scattering at the ice water interface during freezing

The 488 nm radiation of an argon laser is scattered quasi-elastically at the [0001]-face (basal plane) of a growing ice crystal. The scattering plane is the basal plane. The Rayleigh-linewidth is proportional to the square of the scattering vector. One measures about 2 krad/s at a scattering angle of 90°. The linewidth does not depend on the growth rate. Scattering is only observed once a critical growth rate ν_crit = 1,5 μ m?s has been exceeded. Then the scattering intensity depends linearly on the growth rate in the range between 0,03 μ m?s and 2,5 μ m?s. Once the surface is molten scattering vanishes and does not reappear until the growth rate has again exceeded the threshold ν_crit. The coherence properties of the scattered light indicate that the thickness of the scattering layer is less than 6 μ m. The observations are interpreted in terms of a fluctuating interface. The decay time of the fluctuations has been calculated. Satisfactory agreement with the observed Rayleigh linewidth is obtained.     

Field coherence in laser-assisted charged-particle scattering

Summary The problem of charged-particle scattering in the presence of strong laser fields is investigated within a treatment in which the effect of the assisting field is included exactly, to all orders. Two laser field models are considered: 1) the model of a purely coherent field and 2) that of a field with well-stabilized amplitude and fluctuating phase (phase diffusion model, PDM). The scattering cross-sections are numerically investigated for selected but representative values of the parameters of the problem. Differential and total cross-sections are evaluated as functions of the scattering angles, the incident energy, the field coherence and intensity and the number of exchanged photons. The band width of the PDM field is found to affect significantly the scattering parameters, especially in the case of differential cross-sections and of high intensities. As compared to the purely monochromatic (SM) field, the PDM field is found to exhibit oscillations of smaller amplitudes with a tendency to a smooth behaviour with increasing band width, photon multiplicity and intensity. The PDM cross-sections are generally slightly larger than the SM ones. For the first time are reported calculations of the total cross-sectionsvs. the intensity with a sufficiently large density of points to resolve accurately the oscillatory structure of the high-intensity portion of the cross-sections. A consequence ensuing from the exact treatment of the laser field is that the field coherence properties too are treated exactly, at least within the given laser models, and it is expected to be of importance beyond the particular problem treated here, especially when nonresonant processes are concerned.

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Резюме Исследуется проблема рассеяния заряженных частиц в присутствии сильных лазерных полей, причем эффект поля учитывается точно, во всех порядках. Рассматриваются две модели лазерного поля: 1) модель чисто когерентного поля; 2) модель поля с хорошо стабилизированной амплитудой и флуктуирующей фазой. Численно определяются поперечные сечения рассеяния для некоторых значений параметров проблемы. Исследуется зависимость дифференциальных и полных сечений рассеяния от углов рассеяния, падающей энергии, когерентности и интенсивности поля и числа обменных фотонов. Получено, что в рамках модели стабилизированной амплитуды и флуктуирующей фазы полные поперечные сечения почти на два порядка превышают соответствующие поперечные сечения для модели чисто когерентного поля. Отмечается необходимость дополнительных экспериментов. Из проведенного точного анализа лазерного поля следует, что свойства когерентности лазерного поля также должны рассматриваться точно, по крайней мере, в рамках данных лазерных моделей.

     

Dynamic light scattering study of biaxial ordering in a thermotropic liquid crystal

Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals.     

Dynamic light scattering of DNA: Role of the internal motion

Summary An apparatus for dynamic light scattering (DLS) from solutions has been assembled and tested. The technique of DLS appears as a powerful help towards the understanding of the conformation and the dynamics of macromolecules. In the present work circular DNA, PACL29, of 5400 base pairs (molecular weight≃3.27·10⁶ daltons), has been investigated and two components of the diffusion coefficient (D) have been separated. The first corresponding to translational diffusion givesD=(2.9±0.3)·10⁸cm²/s whereas the internal motion corresponds to a diffusion coefficientD _i=(11.7±0.3)·10⁸cm²/s with an internal relaxation time τ_i=(850±200)μs.

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Riassunto è stato messo a punto un apparato per misure di light scattering dinamico (DLS), tecnica di grande utilità per lo studio della conformazione e della dinamica delle macromolecole. In questo lavoro si è studiato un DNA circolare, PACL29, di 5400 paia di basi, peso molecolare 3.27·10⁶ daltons, e si sono separate due componenti esponenziali nella funzione di correlazione ricavando pertanto due coefficienti di diffusione. Il primo corrisponde al moto traslazionale (D=(2.9±0.3)·10⁸cm²/s), mentre ai moti interni corrisponde un coefficiente di diffusioneD _i=(11.7±0.3)·10⁸cm²/s con un tempo di rilassamento interno τ_i=(850±200)μs.

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Резюме Собрана и испытана аппаратура для динамического рассеяния света на растворах. Предложенная техника динамического рассеяния света удобна для исследования конформации и динамики макромолекул. В этой работе исследуется кольцевая ДНК, PACL 29, (молекулярный вес ≃ 3,27 10⁶ Дальтон) и выделяются две компоненты коэффициента диффузии. Первая компонента, соответствующая трансляционной диффузии, даетD=(2.9±0.3)·10⁸ см²/с, тогда как внутреннее движение соответствует коэффициенту диффузииD _i=(11.7±0.3)·10⁸ см²/с с временем внутренней релаксации τ_i=(850±200)μmkc.

     

Raman scattering by localized modes

Raman scattering from the T₂ fundamental and T₂ and A₁ second harmonics of the localized mode for H^- and D^- in CaF₂, SrF₂ and BaF₂ are reported, and are in substantial agreement with theoretical explanations of the i.r. spectra for the same systems.     

Molecular coherence effects in stimulated raman scattering

A semi-classical calculation of the three-level system consisting of the ground state, the vibrationally excited state and the electronic excited state under the laser and the Stokes perturbation is given. The induced molecular polarization produces gain modulation of the Stokes and loss modulation of the laser at a frequency that is dependent on the optical intensity. With the optical intensity in self-trapped filaments in nonlinear liquids such as CS₂, the period of modulation becomes of the order 10^?11 s and a large amplitude modulation of the laser and the Stokes waves will result. The amplitude modulation is not much reduced, if the molecular relaxation time of the order 10^?11 s is taken into account. Effects of non-uniform field distribution and the width and shape of the incident laser pulse are discussed. The frequency broadening caused by the three-level effect is shown to be larger than, or at least as large as, the broadening caused by the optical Kerr effect.     

Dynamic light scattering as a probe of orientational dynamics in confined liquid crystals

The eigenmodes of director orientational fluctuations in nematic liquid crystals in confined geometries were studied both theoretically and experimentally by dynamic light-scattering tehnique. The fundamental mode of the orientational fluctuations shows a crossover from bulk behavior, dominated by bulk elastic constant K, to surface dominated one, in which the relaxation rate is determined by the ratio of surface anchoring strength W and viscosity eta. The contribution of surface viscosity zeta is also significant when its characteristic length zeta/eta becomes comparable to the size of the confined system. It was measured in nematic liquid crystal in cylindrical pores of polycarbonate (Nuclepore) membranes to be of the order of 10 nm.     

Slice selection in solids by localized double-quantum coherence transfer

Double-quantum heteronuclear coherence transfer in solids shows a strong spatial dependence when performed in the presence of a magnetic field gradient. This is a direct consequence of the off-resonance sensitivity of the coherence transfer process and represents a new principle for localized NMR spectroscopy of quadrupole nuclei in solids. Since the slice-selective excitation is achieved simultaneously to the cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic in solids. In the present work, the localization efficiency of this new slice-selection principle was analyzed in dependence on the experimental parameters for a spin system consisting of abundant spin-1/2 and rare spin-1 nuclei. The resulting slice profiles and the calculated dependences of the slice thickness for the basic coherence transfer procedures are discussed on the example of¹H?²H in monodeuterated benzene. The proposed method opens the possibility of volume-selective investigations of the structure and dynamics of materials using the well-established methodology of deuteron-NMR spectroscopy.     

Low-order light scattering in multiple scattering disperse media

Light scattering has been investigated in systems in which both single and higher order scattering occur. The Monte Carlo simulation technique for studying light scattering in randomly inhomogeneous, strongly scattering disperse media was employed. The reliability of the data obtained has been checked by comparing the results of the computer simulation with analytical calculations for the intensity of doubly scattered light. The first several scattering orders have been analyzed for different geometries of the optical experiment. It has been shown, in particular, that, depending on the detector aperture, the contribution of multiple scattering can vary by almost an order of magnitude.