Mandelstam - Brillouin scattering

Mandelstam-Brillouin scattering is the scattering of optical radiation by condensed media (solids and liquids) as a result of its interaction with the natural elastic vibrations of these media. It is accompanied by a change in the set of frequencies (wavelengths) that characterize the radiation — its spectral composition. For example, Mandelstam – Brillouin scattering of monochromatic light leads to the appearance of six frequency components of the scattered light, in liquids - three (one of them is an unchanged frequency). The effect is named after the Soviet physicist Leonid Isaakovich Mandelstam and the French-American physicist Leon Brillouin .

The relatively strong interaction between the particles of condensed matter (it binds them into an ordered spatial lattice) leads to the fact that these particles cannot move independently - any of their excitation propagates in the medium in the form of a wave. However, at any temperature other than absolute zero , the particles are in thermal motion. As a result, elastic waves of various frequencies ( hypersound ) propagate in various directions in the medium. The superposition of such waves on each other causes the appearance of the so-called. fluctuations in the density of the medium (small local deviations of the density from its average value), on which light is scattered . Mandelstam – Brillouin scattering shows that light waves interact directly with elastic waves , which are usually not observed separately.

From the concept of standing waves — condensations and rarefactions of density modulating a light wave — was based on L.I. Mandelstam, who theoretically predicted the scattering of Mandelstam-Brillouin (his article, written in 1918, was published only in 1926). Independently the same results were obtained (1922) by L. Brillouin, considering the scattering of light by elastic waves traveling towards each other in a medium. With his approach to the phenomenon, the Doppler effect is the physical cause of the “splitting” of monochromatic lines.

The first attempts to observe the Mandelstam - Brillouin scattering, made by L.I. Mandelstam and G.S. Landsberg (1930), allowed only to observe the broadening of the Raman scattering lines. The first successful experiments and detailed studies were carried out by E.F. Gross . In particular, he discovered (1938) that Mandelstam-Brillouin scattering splits a monochromatic line into six components (this is explained by the fact that the speed of sound v is different for different directions, as a result of which, in the general case, there are three in it - one longitudinal and two transverse - sound waves of the same frequency, each of which propagates with its own speed v ). He also studied the Mandelstam – Brillouin scattering in liquids and amorphous solids ( 1930 - 1932 ), in which, along with two “shifted” ones, an “unbiased” component of the initial frequency f is also observed. A theoretical explanation of this phenomenon belongs to L.D. Landau and G. Plachek (1934), who showed that, in addition to density fluctuations, it is necessary to take into account fluctuations in the temperature of the medium.