Landau–Placzek ratio is a ratio of the integrated intensity of Rayleigh scattering to the combined integrated intensity of Brillouin scattering of a triplet frequency spectrum of light scattered by homogenous liquids or gases.
The triplet consists of two frequency shifted Brillouin scattering and a central unshifted Rayleigh scattering line split.
The triplet structure was explained by Lev Landau and George Placzek in 1934 in a short publication,[1][2] summarizing major results of their analysis.
Landau and Placzek noted in their short paper that a more detailed discussion will be published later although that paper does not seem to have been published.
However, a detailed discussion is provided in Lev Landau and Evgeny Lifshitz's book.