A model for rotational relaxation and resonance

Depolarized Rayleigh and Raman scattering and far infrared absorption has revealed the rotational motion of polar molecules in liquids. The presence of both a Debye relaxation spectrum and a broad resonance indicates that the molecules oscillate and in addition are subject to some sort of relaxation mechanism. A model is proposed in which the molecule oscillates in a potential well until thermal fluctuations cause it to surpass the potential barrier and jump to a neighbouring well. For two-dimensional rotation approximate analytic expressions are obtained for the spectrum, which agree qualitatively with the measurements. The conclusion is that both resonance and relaxation can be obtained from a single mechanism, so that it is not necessary to introduce one mechanism for each of them.