Linear relaxation times of stochastic processes driven by non-Gaussian noises

The linear relaxation time (LRT) associated with steady-state correlation functions is studied for Langevin equations with non-Gaussian noises: dichotomous Markov noise and Poissonian white shot noise. Exact results for arbitrary models are obtained and compared with results for Gaussian noises. Some general features of LRTs are discussed. The concept of dynamic effective diffusion is introduced and the existence of an optimal effective Fokker-Planck approximation is discussed. Explicit examples for prototype models are presented and briefly compared with the analogs for Gaussian noises.     

A flux-vector splitting method for steady Navier-Stokes equations

The flux-vector splitting method is applied to the convective part of the steady Navier-Stokes equations for incompressible flow. By the use of partial upwind differences in the split first-order part and central differences in the second-order part, a set of discrete equations is obtained which can be solved by vector variants of classical relaxation schemes. It is shown that accurate results can be obtained on one of the GAMM backward-facing step test problems.     

1H relaxation study in (NH4)2SbF5

¹H spin-lattice relaxation rate (T ₁ ⁻¹ ) has been measured using inversion recovery technique in polycrystalline (NH₄)₂SbF₅ system in the temperature range 140–400 K. From the plot of log (M ₀−M) againstτ, we have estimated two differentT ₁ corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT ₁ in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.     

准分子介质中的弛豫振荡(英文)

本文描述了稀有气体卤化物准分子介质中光强的弛豫振荡,使用的高压混合气体由稀有气体.卤素和相应地缓冲(稀有)气体所组成.借助氩离子激光束(514.5nm)探测激活介质,测得三原子准分子Xe_2Cl的弛豫振荡周期值为4nm左右.系统用相对论强电子束进行泵浦.在对准分子介质的光学增益观测中,发现了光场强度弛豫振荡的有趣现象.这种振荡表明了光强与被激励介质间的相互作用.本文首次描述了准分子介质中的这种振荡,其物理学机制可以认为是:光强增加导致受激发射速率增加使得粒子数反转下降,这就引起光学增益减小,而光学增益的减小反过来又导致光强的减弱.我们假设,高压混合气体被电子束泵浦后形成均匀加宽的四能级系统,而激光下能级是空的.利用微扰方法,建立起了描述腔内光子数密度变化R_1的微分方程如下:(?)_1+B(?)_1+Cn_1=D.进一步,类比无线电技术中RLC线路理论,可以得到弛豫振荡的周期T为T=2α[t.τ/(p-1)]~(1/2).其中,p——是泵浦函数,t——是腔内光子寿命,τ——是除受激发射之外,其他一切因素造成的激发态寿命.由这一公式计算出的周期T理论值和实验中观测到的值基本上是一致的.这样,反过来就可由测量弛豫振荡周期,去确定泵浦函数或原子寿命,并和其他方法相佐证.     

分支定界法及其自组织异步并行实现

在讨论分支定界法的并行计算的基础上，就分支定界法求解分段线性规划问题提出了一种具有自组织功能的并行计算过程，并给出了能提高并行效率的异步并行计算的实施方案．     

Comparison of dielectric theories that explicitly include viscoelastic parameters

In this work we compare four dielectric relaxation models that explicitly include the viscoelastic properties of the environment to define the dielectric relaxation process. These models are the Debye, Gamant, DiMarzio-Bishop, and the Havriliak-Havriliak models. Debye's model assumes a Newtonian viscosity and solves the hydrodynamic problem exactly leading to the venerable Debye function. Gamant's used heuristic arguments to include a time-dependent viscosity. These results lead to a splitting of the relaxation process. DiMarzio-Bishop solved the hydrodynamic equations which included a time-dependent viscosity exactly and applied the results to poly(n-octyl methacrylate). The Havriliak-Havriliak approach is based on statistical mechanics arguments that take into account strain energy which is associated with the polarization process. This model also requires a knowledge of the viscoelastic properties of the system. The results of the four models are compared. © 1995 John Wiley & Sons, Inc.     

Activation volumes associated with chromophore reorientation in corona poled guest–host and side-chain polymers

The decay of the second-order optical susceptibility χ⁽²⁾ as a function of temperature and pressure has been studied in a variety of corona poled guest–host and side-chain polymeric materials using second harmonic generation (SHG). The specific systems studied include the side-chain copolymer poly(disperse red 1 methacrylate-co-methyl methacrylate) (DR1-MMA) as well as the series of guest–host materials formed by individually dissolving the dyes Disperse Red 1 (DR1), Disperse Orange 3 (DO3), and N,N dimethyl-p-nitroaniline (DpNA) in poly(methyl methacrylate) (PMMA), polycarbonate (PC), and polystyrene (PS). In each of these systems, the observed relaxation of χ⁽²⁾ can be represented by a Kohlrausch-Williams-Watts stretched exponential, from which the decay time τ and decay distribution width β are determined. For pressures up to approximately 1000 atm, the natural log of the pressure shift factor is seen to vary linearly with applied pressure, yielding the activation volume for rotational reorientation of the chromophores in each system. The activation volumes are loosely correlated with dopant size in a given polymer host, but are not the same for a given dopant in different hosts. Modeling the chromophores as rotating cylinders, we show that the measured activation volumes do not correspond to the average volume swept out by the dye molecules as they reorient. On the other hand, the activation volumes for each of the three dyes dissolved in PS are seen to be in agreement with the measured activation volumes for the molecular motions associated with volume recovery in neat PS. Moreover, the activation volumes for DR1 and DpNA dissolved in PS are seen to correlate with the proposed couplings between the rotational reorientation of DR1 and the α-relaxation dynamics of PS and the slight decoupling of DpNA from the α-transition motion of PS. This correlation suggests a possible relationship between the activation volumes for chromophore reorientation and the size of the components of the host polymer or the volume swept through by the polymer components during structural reconfiguration. We demonstrate that assuming activation volumes for chromophore reorientation to be related to the size or motion of the polymer host constituents yields a consistent interpretation of the observed trends in the measured activation volumes. © 1995 John Wiley & Sons, Inc.     

Electron spin relaxation of radicals in weak magnetic fields

The review summarizes the results of studies on specific features of spin relaxation of radicals in liquids in weak magnetic fields. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1642–1654, October, 2006.     

A Newton/upwind method and numerical study of shock wave/boundary layer interactions

The objective of the paper is twofold. First we describe an upwind/central differencing method for solving the steady Navier–Stokes equations. The symmetric line relaxation method is used to solve the resulting algebraic system to achieve high computational efficiency. The grid spacings used in the calculations are determined from the triple-deck theory, in terms of Mach and Reynolds numbers and other flow parameters. Thus the accuracy of the numerical solutions is improved by comparing them with experimental, analytical and other computational results. Secondly we proceed to study numerically the shock wave/boundary layer interactions in detail, with special attention given to the flow separation. The concept of free interaction is confirmed. Although the separated region varies with Mach and Reynolds numbers, we find that the transverse velocity component behind the incident shock, which has not been identified heretofore, is also an important parameter. A small change of this quantity is sufficient to eliminate the flow separation entirely.     

Electronic Relaxation in Indium Oxide Films Studied with Perturbed Angular Correlations

The electronic relaxation due to the ¹¹¹In → ¹¹¹Cd electron capture process was studied by measuring perturbed γγ-angular correlations after ¹¹¹In-ion implantation into indium oxide films of various compositions and thicknesses and deposited on different substrates. X-ray diffraction, Rutherford backscattering and secondary ion mass spectrometry were used to characterize the microstructure and composition of the samples. The film thickness and type of substrate were found to have much less influence on the after effect than the deposition parameters and impurities.