Dynamics of density fluctuations. Inelastic light scattering

Dynamics of density fluctuations. Inelastic light scattering 

The study of inelastic light scattering (the spectral dependence of scattered light) has led to a concept of two kinds of density fluctuations, namely, an adiabatic one and an isobaric one (Landau and Placzek, 1934 Gross, 1940, 1946 Fabelinski, 1965 Kerker, 1969 Vuks, 1977). 

An clastic wave propagates with the velocity V, and Doppler s shift in frequency must occur. Of all possible directions of elastic waves, only two meet condition 17 those which propagate along two opposite directions with the velocities +V and - V, so the spectrum of scattered light is split into Mandelshtam-Drillouin s doublet (Fabelinski, 1965 Kerker, 1969 Vuks, 1977) 

Isobaric density fluctuations irc due to the entropy fluctuations, i.e. fluctuations in Frenkel s holes in the structure of a liquid (Frenkel, 1955). They lead to the density fluctuations which do not depend on pressure fluctuations. Their evolution proceeds quite slowly and causes a much lower frequency shift on scattering. These fluctuations are related to the central component of scattered light called Rayleigh s component. 

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