Effect of guest-host interaction on the dynamics of ethylene glycol in H-ZSM5 zeolite

Dynamics of ethylene glycol (EG) adsorbed in H-ZSM5 zeolite as studied using quasielastic neutron scattering (QENS) technique is reported here. Analysis of QENS data revealed that observed dynamics correspond to isotropic rotational motion of EG. Rotational diffusion coefficient of EG adsorbed in H-ZSM5 zeolite is found to be slower compared to bulk and comparable to that of benzene adsorbed in the same host. Positron Annihilation Spectroscopy (PAS) technique is used to understand the nature of guest-host interaction and the results of both QENS and PAS are found to be consistent with each other.     

Diffusion of water in nano-porous polyamide membranes: Quasielastic neutron scattering study

Dynamics of water sorbed in a reverse osmosis polyamide membrane (ROPM) as studied by quasielastic neutron scattering (QENS) is reported here. The trimesoylchloride-m-phenylene diamine based ROPM is synthesized by interfacial polymerization technique. QENS data indicates that translational motion of water confined in ROPM gets modified compared to bulk water whereas rotational motion remains unaltered. Translational motion of water in ROPM is found to follow random jump diffusion with lower diffusivity compared to bulk water. Translational diffusivity does not show the Arrhenius behaviour.     

Rotational dynamics of propylene inside Na-Y zeolite cages

We report here the quasielastic neutron scattering (QENS) studies on the dynamics of propylene inside Na-Y zeolite using triple axis spectrometer (TAS) at Dhruva reactor, Trombay. Molecular dynamics (MD) simulations performed on the system had shown that the rotational motion involves energy larger than that involved in the translational motion. Therefore, rotational motion was not observed in our earlier QENS studies on propylene adsorbed Na-Y zeolite using a higher resolution spectrometer at Dhruva. Analysis of the TAS spectra revealed that the quasielastic broadening observed in propylene-loaded zeolite spectra is due to the rotational motion of the propylene molecules. This is consistent with our simulation result. Further, the rotational motion is found to be isotropic. The rotational diffusion coefficient has been obtained.      

Phase separation in binary systems with competing internal and external multiplicative noises

We have considered phase separation processes in binary stochastic systems with thermal diffusion and ballistic mixing representing irradiation influence. Introducing fluctuations of thermal flux and an external source of atom relocations due to ballistic diffusion into dynamics of a globally conserved field, we have shown that there are two competing mechanisms of phase transitions type of “order-disorder”: thermally assisted diffusion and irradiation induced atomic exchange. We have studied dynamics of the structure function at early stages of decomposition. In the framework of the mean field theory we have derived the effective Fokker-Planck equation to describe phase separation processes. It was shown that the ordering processes can be controlled by both regular and stochastic parts of external source influence. A reentrant behavior of a mean field order parameter versus the external noise intensity and fluctuations correlation radius is found.     

Influence of the phase coherence length on ballistic transport

The characteristic length scales for the transport in disordered metals are discussed. Based on a phenomenological model of phase randomising scattering processes, the influence of the phase coherence length on the conductance of ballistic systems is studied. It is argued that the frequency dependence of the conductance of quasi-one dimensional systems can be used in order to determine not only the statistical average but the whole distribution function of the phase coherence length. Various cases of distributions, the -function, the exponential, and the Gamma distribution, are discussed. It is shown that due to quantum coherence effects deviations from the classical (Drude) behavior of the conductance exist. For independent scattering processes the probability distribution function is given by the Poisson distribution function. In this case an expression for the conductance can be derived which contains the ballistic transport, and the result for the exponential distribution.     

Acetylene diffusion in Na-Y zeolite

Study of diffusivity of acetylene adsorbed in Na-Y zeolite by quasi-elastic neutron scattering (QENS) measurements at temperatures of 300, 325 and 350 K is reported. A model in which the acetylene molecules undergo random-walk diffusion characterized by a Gaussian distribution of jump lengths inside zeolite cages describes the data consistently. The diffusion constant, residence time between jumps and root mean square jump length are determined.     

水泥老化过程中水动态的准弹性中子散射(QENS)谱分析

准弹性中子散射(quasi-elastic neutron scattering, QENS)实验是研究水泥老化过程中水动态的一种新颖的实验方法.本文利用老化时间分别为7, 14和30 d水泥样品的QENS谱实验数据, 通过应用四个高斯项的和的能量分辨函数R(Q, E)代替一个高斯项的能量分辨函数来改进经验扩散模型(empirical diffusion model, EDM), 再进行非线性最小二乘拟合.由此导出水泥样品中水动态的相关物理参数: 不动水数密度A, 自由水指数FWI=B₁/(A+B₁+B₂), 洛伦兹函数的半高宽Γ, 移动水跳跃之间的平均停留时间τ ₀及自扩散系数D_t, 而且可得出更为精准的QENS谱拟合曲线.拟合得到的物理参数可定量描述水泥老化过程中水动态过程, 从而为QENS实验在水泥老化过程中水动态研究的应用提供一种合理实用的谱分析方法.     

基于准弹性中子散射谱分析水化硅酸钙(C-S-H)中受限水的动态

研究水化硅酸钙(C-S-H)中受限水动态的一种重要手段是准弹性中子散射(quasi-elastic neutron scattering, QENS)实验. C-S-H样品的QENS谱数据可通过跳跃扩散和转动扩散模型进行分析拟合, 进而导出C-S-H样品微纳孔中水动态的相关物理参数: 不动水指数C、转动扩散系数D_r、均方位移< u² >、自扩散系数D_t及平均停留时间τ₀. 本文对水与水泥质量比为30%的C-S-H样品, 测量温度为230-280 K的QENS谱进行了分析, 得到的拟合参数可定量描述C-S-H样品内不同尺度的微纳孔中受限水随温度变化的动态过程. 转动扩散系数D_r随散射矢量Q的变化可知, Q值较大时, 水分子的转动对QENS谱影响较大. 均方位移 < u² > 随Q值的不同而变化, 其拟合值可区分C-S-H样品中的不动水、强受限水和受限水. 在Q较小时, D_t 和τ₀ 的拟合值随温度而变化, 并分别在230和240 K突然增大, 由此揭示温度为230-240 K 时, C-S-H 样品中受限水分子的动态特性发生了转变.     

Mode dependent scattering of phonons by domain walls in ferroelectric KDP

Thermal conductivity and ballistic phonon imaging measurements in KH₂PO₄ (KDP) at low temperature (T<3K) indicate that scattering from domain walls has a large effect on phonon transport. kDP has a ferroelectric phase transition from tetragonal to orthorhombic structure atT _c =122 K. BelowT _c domains of opposite electric polarization and crystal orientation form unless the sample is colled in an electric field. Thermal conductivity measured along the [100] (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. We attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domainwall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission.     

Translational diffusion of water in compacted clay systems

The translational diffusion of water in compacted clays at a high hydration level has been investigated by quasielastic neutron scattering at a time-of-flight spectrometer FOCUS (SINQ). Four compacted clays with systematic structural differences have been studied: Na-montmorillonite, Na-illite, kaolinite and pyrophyllite. The QENS experiments were performed using two different incident wavelengths in order to access a larger Q range and verify the data analysis. The translational diffusion coefficient for water in Na-montmorillonite and Na-illite are lower than those for bulk water, whereas the preliminary results for kaolinite and pyrophyllite show larger diffusion coefficient.     

紫外波段多分散系气溶胶散射相函数随机抽样方法研究

基于电磁散射与辐射传输中的基本理论,对紫外波段霾尺度范围内满足特定分布的多种气溶胶粒子的散射相函数进行了研究.提出了一种直接随机抽样拟合散射相函数的方法.比较了H-G相函数、改进的H-G相函数及随机抽样拟合的相函数与多分散系Mie相函数的偏离程度.数值计算了不同相函数拟合方法对应气溶胶的传输特性.计算结果表明,相函数的准确模拟计算对于蒙特卡罗方法等辐射传输问题的解决具有十分重要的意义.     

Experimental evidence of selective heating of molecules adsorbed in nanopores under microwave radiation

We have performed in situ quasielastic neutron scattering (QENS) measurements on zeolite-guest systems under microwave irradiation, for comparison with corresponding simulations. Both experiment and simulation reveal selective heating of methanol in silicalite, but little to no heating of benzene in silicalite. Effective translational and rotational temperatures extracted from QENS data under microwave heating were found to depend on microwave power. In agreement with simulation, QENS rotational temperatures significantly exceed translational ones at high microwave power, thus providing the first microscopic proof for athermal effects in microwave-driven nanopores.     

水中密排圆柱体群的超声多重散射参数

以水中紧密排列的平行圆柱体群为对象,研究平面超声脉冲经多重散射后的透射波性质,通过分析其中头波和散射波的特征获得对应的多重散射参数.对直径随机分布、位置无序排列、数量密度约100个/cm²、面积占空比约0.53的非接触圆柱体群,采用中心频率2.5 MHz的宽带脉冲波入射。为解决透射信号在时域表现出随机性的问题,将散射体尺寸、分布都相同但位置分布不同的多个模型仿真的透射波叠加平均后用于分析.在频域对头波的宽带衰减系数进行分析,并在时域研究散射波声强的时间演化曲线,获得了系统的弹性平均自由程、传输平均自由程等多重散射参数。经多重散射后,透射波中的头波表现出相干性,由不相干近似理论可对其对应的散射参数进行定性描述;散射波是不相干的,其对应的多重散射参数可近似利用扩散近似理论获得。     

Diffusion of propylene adsorbed in Na-Y and Na-ZSM5 zeolites: Neutron scattering and FTIR studies

Here we report the quasielastic neutron scattering and FTIR studies on the dynamics of propylene adsorbed in Na-Y and Na-ZSM5 zeolites. QENS data show that although the mechanism of translational motion of propylene is jump diffusion in both the cases of Na-Y and Na-ZSM5 zeolites, the diffusivity is affected by the host size and is hindered in the case of Na-ZSM5. FTIR studies showed that guest-host interaction in Na-ZSM5 is stronger than that in Na-Y zeolite corroborating the QENS results.      

Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations. An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.      

Excess water dynamics in hydrotalcite: QENS study

Results of the quasi-elastic neutron scattering (QENS) measurements on the dynamics of excess water in hydrotalcite sample with varied content of excess water are reported. Translational motion of excess water can be best described by random translational jump diffusion model. The observed increase in translational diffusivity with increase in the amount of excess water is attributed to the change in binding of the water molecules to the host layer.     

Interior radiances in optically deep absorbing media—III. Scattering from haze L

The interior radiances are calculated within an optically deep absorbing medium scattering according to the Haze L phase function. The dependence on the solar zenith angle, the single scattering albedo, and the optical depth within the medium is calculated by the matrix operator method. The development of the asymptotic angular distribution of the radiance in the diffusion region is illustrated through a number of examples; it depends only on the single scattering albedo and on the phase function for single scattering. The exact values of the radiance in the diffusion region are compared with values calculated from the approximate equations proposed by Van de Hulst. The variation of the radiance near the lower boundary of an optically thick medium is illustrated with examples. The attenuation length is calculated for various single scattering albedos and compared with the corresponding values for Rayleigh scattering. The ratio of the upward to the downward flux is found to be remarkably constant within the medium. The heating rate is calculated and found to have a maximum value at an optical depth of two within a Haze L layer when the sun is at the zenith. The location of this maximum moves toward the top of the haze layer as the solar zenith angle increases and also as the single scattering albedo decreases. When the single scattering albedo is less than 0·8, the downward flux is so small within the diffusion region that experimental measurements are probably not possible.     

Quasi-elastic neutron scattering study of dynamics in condensed matter

Quasi-elastic neutron scattering (QENS) technique, known to study stochastic motions has been successfully used to elucidate the molecular motions and physical properties related to them, in a variety of systems. QENS is a unique technique that provides information on the time-scale of the motion as well as the geometry of the motions. In this paper, results of some of the systems studied using the facility available at Dhruva, Trombay and other mega-facilities are discussed. Emphasis is given on the results obtained from three different systems studied using QENS, namely, (1) alkyl chain motions in monolayer protected metal clusters, (2) molecular motions of propane in Na-Y zeolitic systems and (3) the study of reorientational motions of liquid crystal innO.m series in different mesophases.     

Emergent intensity from a plane-parallel medium exposed to a Gaussian laser beam: A comparison of Rayleigh and isotropic scattering

The focus of this two-dimensional study is the radially varying intensity emergent from a plane-parallel scattering medium exposed to a collimated, Gaussian laser beam directed perpendicular to the upper surface. The method of analysis is the integral transform technique. Specifically, this work uses the generalized reflection and transmission functions from a previous study to construct the emergent intensity with the use of an inverse Hankel transform. Radially varying backscattered and transmitted intensities are calculated for media with isotropic and Rayleigh scattering phase functions and optical thicknesses that range from 0.125 to 8.0. The behavior of the emergent radiation inside and outside the beam is investigated for both narrow and wide beams. A new integration method is implemented to compute the emergent intensity at the beam center. The emergent intensity at the beam center is used to quantify when a one-dimensional model may be used. As expected, for small optical thicknesses and near the beam the phase function has significant influence, while far from the beam multiple scattering reduces the influence of the Rayleigh phase function. Results from this study will be useful in understanding and interpreting more complicated situations, such as those that include polarization.     

Estimation of a low-order Legendre expanded phase function of snow

The purpose of this paper is to estimate the scattering phase function of snow from angularly resolved measurements of light intensity in the plane of incidence. A solver is implemented that solves the scattering function for a semi-infinite geometry based on the radiative transfer equation (RTE). Two types of phase functions are considered. The first type is the general phase function based on a low-order series expansion of Legendre polynomials and the other type is the Henyey–Greenstein (HG) phase function. The measurements were performed at a wavelength of 1310 nm and six different snow samples were analysed. It was found that a first order expansion provides sufficient approximation to the measurements. The fit from the first order phase function outperforms that of the HG phase function in terms of accuracy, ease of implementation and computation time. Furthermore, a correlation between the magnitude of the first order component and the age of the snow was found. We believe that these findings may complement present non-contact detection techniques used to determine snow properties.