Negatively modified scattering

Abstract

In a paper published in 1923, Einstein and Ehrenfest discussed the general problem of thermodynamical equilibrium in an assembly traversed by light, which is subjected to scattering by the particles of the assembly. They deduced, in a way (which is quite analogous to Einstein's famous deduction of the laws of black body radiation by consideration of thermodynamical equilibrium in the presence of Bohrtype of vibrators), that the excited particles of the system would give rise to a type of modified scattering analogous to Einstein's negative absorption. Smekal pointed out in a letter to Naturwissenschaften (1923) that if light of frequency v is scattered by the particles of the assembly, then in addition to unmodified scattering of frequency v, scattered light should also consist of light of frequency v+v_k, v-v_k, where hv_k is the energy difference between the excited state and the normal state of the scattering particles ("Handbuch der Physik," Bd. 23, pp. 93-99).