Perturbation theory for equilibrium properties of molecular fluids

Perturbation theory for the angular pair correlation function g(r₁₂ ω ₁ ω ₂), using a fluid with isotropic intermolecular forces as the reference system, is applied to the calculation of a variety of macroscopic properties. Comparisons with experiment are made for methane, oxygen and nitrogen (and carbon monoxide for infra-red and Raman band moments) in the dense fluid and liquid states. Theoretical expressions are given and calculations made for thermodynamic properties (isothermal compressibility, pressure, configurational energy, entropy and specific heat) both along and away from the vapour-liquid co-existence curve, for infra-red and Raman band moments, and for neutron scattering cross sections. Excellent agreement with experiment is obtained for all properties, except for the infra-red and Raman band moments; this latter comparison is inconclusive because of large experimental uncertainties. The anisotropic intermolecular forces are found to have very little effect on the liquid isothermal compressibility, in agreement with the first-order theory. Molecular anisotropy has a relatively small effect on the configurational energy and on the Helmholtz free energy, but the effect is large for pressure and specific heat. The pressure is more sensitive to short-range anisotropic forces than the other properties, whereas the specific heat is particularly sensitive to the long-range anisotropic forces. Mean squared torques (derived from infra-red and Raman band moments) are very sensitive to the strengths of the anisotropic forces, and are more sensitive to higher terms in the multipole series than are the other properties. The structure factors for oxygen and nitrogen are found to be little affected by the anisotropic forces.     

Features of the shape of Raman spectra of disordered ferroelectrics

A theory of the shape of the first-order Raman scattering (FORS) line is developed in which nonlinear and correlation effects in inhomogeneous broadening and the dynamic mechanism of homogeneous broadening are taken into account. It is shown that the FORS band generally has two peaks and that the shape of the low-frequency peak is determined by homogeneous broadening. Our theory provides an explanation for the features of the observed FORS band of the hard TO ₂ mode in KLT and KTN with various concentrations of Li and Nb ions.     

Theoretical study of Raman scattering from pure liquids

A theory is presented to study the Raman spectra of liquids formed by diatomic molecules in the fundamental spectral region. This theory is based on the fact that the Hamiltonian of a liquid sample is invariant under the operations of the full N-symmetric group. It is found, in particular, that the isotropic Raman scattering is a partially coherent and, if the vibrational rotational correlation is neglected, the anisotropic Raman scattering is a totally incoherent process.     

Reorientational motion in binary mixtures: molecular dynamics simulations

Molecular reorientational motion has been studied in a dilute solution of linear ‘tracer’ molecules in a solvent that exhibits liquid, plastic and crystalline phases. Molecular dynamics simulations have been used to extract reorientational correlation functions for both solvent and solute species as functions of temperature in all phases. The transition from the liquid to the plastic phase (upon cooling) results in less hindered tracer rotation, as evidenced by the more rapid decay of orientational correlation. These surprising dynamics are interpreted in terms of structural changes at freezing that lead to a less confining local tracer environment. The findings support a recent experimental result obtained from polarized Raman scattering on a solution of CS₂ tracers in a cyclohexane host.     

Raman studies on local structural disorder in silicon-based amorphous semiconductor films

Raman scattering and ESR measurements were carried out for amorphous Si and amorphous Si-based alloy systems such as Si-N and Si-C. A pronounced broadening of the Si-Si To-like band of Raman spectrum is observed with an increase in the N or C content for amorphous Si-N and Si-C alloy systems. The broadening of the TO-like band for amorphous Si system as well as amorphous Si-N and Si-C systems has a good correlation with an increase in the density of dangling bonds derived from ESR measurements. The fact suggests that dangling bonds are produced in order to relax the local strain due to structural constraint and lower the total energy.     

GaN直纳米线的光学性能

对 Ga N直纳米线的拉曼光谱及光致发光光谱进行了研究。拉曼光谱表明 ,与计算值相比 ,E2 ( high)声子频率在 560 cm- 1有 -9cm- 1的移动 ,这种声子频率显示出向低能带频移及带变宽的特征 ,是由于纳米尺寸效应所引起的结果。体系的光致发光光谱在 3 44 .8nm附近的近带隙发光 ,与文献报道的 Ga N体材料的数值3 65nm相比有一蓝移 ,这是由于量子限制效应造成的     

Interplay between phonon confinement effect and anharmonicity in silicon nanowires

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.     

Phosphorescence from the two triplet states of p-benzoquinone and toluquinone vapour

Molecular formulae for the isotropic and anisotropic scattering are derived by considering the internal field acting upon a molecule within a dense medium and its fluctuation caused by density fluctuations. The anisotropic scattering formula of classical theory is retained but modified by the factors of the internal field. The new formula for the isotropic scattering depends sensitively on the principal values of the optical polarizability tensor and on the parameters describing the anisotropic internal field. Assuming the internal field to be given by the semimacroscopic approach of the Onsager-Scholte model with an ellipsoidal cavity, comparison of the calculated isotropic Rayleigh factor with the experimental value allows a prediction of the degree of the cavity anisotropy for different liquid densities.     

On the form of the Raman tensor of normal modes in electronically degenerate states

A theory is proposed to derive the form of the scattering tensor of normal modes of electronically degenerate states. Essentially group theoretical arguments are used and emphasis is placed on the origin of asymmetric tensors of Raman and Rayleigh processes.     

Integrated spectral and spatial information extraction in Raman spectroscopy

Raman spectroscopy has been widely used to analyze various substances quantitatively. Conventional studies are primarily focused on the spectral characteristics of Raman scattering. The spatial distribution is always ignored, which can be used to observe the physical properties, such as the particle size. In this article, the spatial information has been extracted from the Raman spectra of barium nitrate, demonstrating that the evident spatial width broadening varied with the particle size. The numerical result shows that the spatial width has a better linear correlation with the particle size, while the Raman intensity has a poor linear correlation. The integrated spectral and spatial information extracted in Raman spectroscopy has a potential application in the quantitative analysis of physical properties.     

Diffusion Collisionnelle Induite Pour Les Bandes Rayleigh Et ν1 Raman Du Liquide Isotopique CCl4

We have studied the depolarized component of the ν₁ Raman band of the isotopic liquid C³⁵Cl₄ at 296 K. We compare the experimental Rayleigh and Raman depolarized scattering with theoretical spectra calculated using dipole induced dipole approximation.     

Many-body correlations for Raman and Rayleigh collision induced scattering

In this paper, the spectral shape of the collision induced scattering is calculated in the dipole-induced-dipole approximation for the Raman symmetric vibrational band of optically isotropic molecules. Comparison is made with Rayleigh results. Experimental data for Rayleigh and v ₁ Raman band of CF₄ are discussed.     

Bimodal dependence of light scattering/fluctuations on the concentration of aqueous solutions

Two concentration ranges (from 10^?5 to 10^?9 and from 10^?13 to 10^?18 M) corresponding to enhanced fluctuations of Rayleigh and Raman scattering of second-harmonic (527 nm) pulses of YVO₄:Nd³⁺ laser are found for aqueous solutions of antioxidant potassium phenosan. A correlation is revealed between the rise in elastic Rayleigh scattering intensity and its fluctuations and the shift of the center of OH Raman band of water toward the ice component characteristic frequency (3200 cm^?1). The development of phase-equilibrium instabilities is analyzed based on the model of fluctuations of the number of hydrogen bonds on the assumption of formation/destruction of ordered hydration layer of phenosan molecules in water.     

Depolarized light scattering in a 2.6-lutidine-water solution near the critical point

The spectra of depolarized Rayleigh light scattering, i.e., the Rayleigh line wing (RLW), and Raman scattering in a solution with the lower critical point in the phase separation diagram is experimentally studied. The Rayleigh line wing is studied to 30 cm^?1 from the exciting light frequency; the Raman scattering line (RSL) is studied in the near profile region. In the vicinity of the lower critical point, as well as in solutions with the upper critical point, strong narrowing of the RLW and RSL is observed. In this case, the conditions arise for the manifestation of the profile in the Rayleigh line spectrum, which is caused by the interaction of concentration and anisotropy fluctuations. Previously, this spectral line was theoretically predicted by I. A. Chaban in [1].     

温度对β胡萝卜素结构有序的影响

本文测量了全反式β胡萝卜素在二甲基亚砜中81–25 ℃ 范围的紫外–可见吸收和拉曼光谱. 结果表明, 随温度降低, 紫外–可见吸收光谱、拉曼光谱都发生红移, 拉曼光谱线型变窄, 散射截面增加这些现象的发生是由于随温度降低, β胡萝卜素分子的热无序降低、分子结构有序性增加、π电子离域扩展, 有效共轭长度增加, 分子的电子能隙变窄. 另外, 随着温度的降低, 溶剂密度增加, 由Lorentz-Lorenz 关系得知相伴的折射率增加, 从而引起吸收光谱的红移. CC键键长增加, 使CC 键拉曼光谱红移; 振动弛豫时间变长, 各CC 键之间的键长差减小, 线宽变窄; 但由于声子, π电子耦合加强使CC键拉曼线型不对称程度增加, 低频端"肩"扩展, CC键的弱阻尼相干振动增加, 使拉曼散射截面增加. 关键词： β胡萝卜素 分子结构有序 红移 拉曼散射截面     

The Raman noncoincidence effect of the ν(CO) band of ME6N liquid crystal across the nematic–isotropic phase transition studied by a micro‐spectroscopy experiment

The Raman spectroscopic noncoincidence effect (NCE) of the ν(CO) band of the liquid crystal ME6N (4‐cyanophenyl‐4′‐hexylbenzoate) has been measured at different temperatures (47–52 °C) around the nematic‐isotropic phase transition (47.8 °C) employing a micro‐Raman experiment under confocal conditions and performed on a homogeneously aligned thin sample. The low value of NCE (0.9 cm⁻¹) obtained over the whole temperature range suggests that the orientational structure of the liquid crystal in both phases is governed by the steric hindrances in the proximity of the carbonyl group, rather than by dipolar interactions. This hypothesis is supported by the results of a supplementary investigation of the NCE of the ν(CO) Raman band in liquid ketones and esters, made progressively more hampered by the insertion of bulky (phenyl) groups in proximity of the carbonyl group. The NCE of the ν(CO) band, in fact, decreases from 5.5 cm⁻¹ in acetone (the less hampered) to 0.7 cm⁻¹ in benzophenone (the most hampered among the studied ketones), and from 6.2 cm⁻¹ in methyl acetate (the less hampered) to 2.2 cm⁻¹ in phenyl benzoate (the most hampered among the studied esters). To our best knowledge, this represents the first attempt to analyze the NCE in terms of steric hindrance of the substituents around the target oscillator. A parallel analysis of the difference between the anisotropic and the isotropic bandwidths of the ν(CO) Raman band in these molecular liquids indicates that reorientational dynamics plays only a marginal role, if any. Copyright © 2006 John Wiley & Sons, Ltd.     

Cascaded Raman generation in optical fibers: influence of chromatic dispersion and Rayleigh backscattering

We study experimentally the influence of chromatic dispersion and Rayleigh backscattering on cascaded Raman generation in silica optical fibers. Effects ranging from enhanced spectral broadening of the Stokes orders to generation of higher Stokes order at unexpected wavelengths are observed. Additionally, we show that four-wave-mixing processes can quench the noisy Rayleigh lasing lines generated in power Raman amplifiers. Our observations are confirmed by numerical simulations.     

Evidence of the presence of opticlike collective modes in a liquid from neutron scattering experiments

Inelastic neutron scattering data from liquid DF close to the melting point show, in addition to spectra comprising quasielastic and heavily damped acoustic motions, an intense, nondispersive band centered at about 27 meV along with a broader higher energy feature. Observation of the former band provides the first direct verification of the existence within the liquid state of collective opticlike excitations as predicted by molecular dynamics simulations. The latter corresponds to mainly reorientational motions assigned from mode eigenvector analysis carried out by computer simulations.     

Quantum kinetic theory of polyatomic gases

A review is given of the kinetic theory of polyatomic gases as based on the quantum mechanical Waldmann-Snider kinetic equation. The transport properties in the Navier-Stokes and Burnett regimes and their dependence on external electric and magnetic fields (Senftleben and Senftleben-Beenakker effects) as well as flow and heat-flow birefringence are discussed for gases consisting of linear, spherical top and symmetric top molecules. The relaxation phenomena associated with sound propagation, Rayleigh-Brillouin, depolarized Rayleigh and Raman light scattering are considered in some detail as well as nuclear magnetic and electron spin relaxation and pressure broadening of microwave spectral lines. Finally an overview is given of the quantum mechanical methods for the quantitative calculation of all these phenomena from the nonspherical intermolecular potential.     

一种用多色光作为激发源产生拉曼光谱的新算法

探讨用多色光代替激光作为拉曼光源的新型拉曼光谱仪的可能性。根据拉曼光谱原理, 并通过数学分析, 发现当用多色光照射在样品上时, 所得到的散射光经散射频率校正后在不同频率上的强度分布可以写成样品的Raman-Rayleigh联合散射谱和激发光光源的功率谱的卷积。利用傅里叶变换算法,有可能从多色光照射样品所得到的散射光谱中导出样品的拉曼光谱。基于上述原理,可能发展出不用激光的新一代拉曼光谱仪。