Surveys the developments in Brillouin scattering during the past fifteen years. Mainly classical effects are described, therefore excluding stimulated and resonance Brillouin scattering. The study is limited so that the frequency interval of the acoustic excitations lies in the hypersonic regime.

https://iopscience.iop.org/article/10.1088/0034-4885/45/3/002/pdf

Brillouin scattering in condensed matter

The long tails at high Stokes and anti-Stokes frequencies observed in Rayleigh scattering from compressed methane, carbon tetrafluoride and sulphur hexafluoride are attributed to induced rotational transitions. The longest-range contribution to the collision-induced polarizability, the dipoleinduced-dipole term in R -3, where R is the separation of a pair of molecules, does not fluctuate with molecular rotation. However, the dipole-quadrupole polarizability A of the tetrahedron leads to a rotating contribution in R -4. The dipole-octopole polarizability E of an octahedron and a tetrahedron leads to a contribution in R -5 which also rotates with the molecule. The rotational transitions associated with these polarizabilities can explain both the intensity and the shape of the long tails. Approximate values of A for CH4 and CF4 and of E for SF6 are deduced. As in the monatomic case, there is a simple relationship between the total scattered intensity and the second Kerr virial coefficient.

Collision-induced rotational Raman scattering by tetrahedral and octahedral molecules

High frequency sound propagation in water has been studied by Brillouin scattering in the entire temperature range of −9°C⩽t⩽100°C. Sound speeds deduced from the data show a peak at t≃70°C in agreement with previous ultrasonic results. Damping constants measured in a temperature range −9°C⩽t⩽20°C are also consistent with the ultrasonic values whenever available. A small negative dispersion of sound velocity is found however in the range 0°C⩽t⩽15°C. We explain this by a simple relaxation model similar to that used by Boon and Fleury in the case of rare gas liquids. We also use a theory of Brillouin scattering which takes into account the temperature fluctuation, and find a small correction term to the expression of the Landau–Placzek ratio. The correction term is calculated and is shown to be nonnegligible for water near 4°C. We believe the data taken in the supercooled regime are reported for the first time and their steep temperature dependence is as yet to be understood theoretically.

Brillouin scattering studies of normal and supercooled water

Photomultiplier tubes which perform virtually ideal digital detection of electromagnetic radiation at visible frequencies are now widely available. The only sources of noise are a generally low dark count, typically a few counts per second, and the unavoidable shot noise due to the random (Poisson) nature of the detection process. The output of such detectors consists of a series of discrete pulses of charge, each corresponding to a single photodetection, which constitutes a Poisson process rate-modulated by variations in the incident intensity. Coincident with the development of these detectors has been the development of hardware based on integrated circuits which can perform errorless analysis of digital signals at frequencies up to about 108 Hz. Combination of these two technologies provides the techniques of photon correlation.1 (We will include the study of single-interval photon statistics in the generic term “photon correlation”.) With speed and accuracy close to the theoretical limit these techniques can be used to analyze fluctuating optical signals of any origin (provided the fluctuation time is ≳10−7 sec).

Statistical Properties of Scattered Radiation

Recent advances are making it possible for optical oceanographers to solve a host of pressing environmental problems.

/pdf/shedding-new-light-on-light-in-the-ocean-v015bs41zb.pdf

Shedding new light on light in the ocean

Generalized hydrodynamics suggests that at high frequencies, there should be significant departire from classical hydrodynamics behavior in simple fluids. In particular, the frequency dependence of transport coefficients is not negligible and should introduced observable effects into the propagation of hyper-sound. This work which analyzes rayleigh-Brillouin spectra measured in liquid Argon and Neon reports one of the first experimental investigations of generalized hydrodynamic behavior in simple liquids

/pdf/laser-light-scattering-in-fluid-systems-3vg3t0ldfs.pdf

Ilya Prigogine

Papers

Laser light scattering in fluid systems