Dielectric response and a phenomenon of a narrow band absorption for a classical rotor in a double well potential

Abstract

The theory of dielectric relaxation in a planar ensemble of polar molecules is presented for a model where dipoles may rotate in a conservative double well potential (DWP) U=U0 sin2 ϑ. The evolution of wideband dielectric spectra as a function of the potential well depth U0 is studied; both isotropic and anisotropic media being taken as examples. The spectra comprise the Debye relaxation and the quasiresonant Poley absorption regions. The exact theory is compared with a simple quasielastic bond/extended diffusion approximation. The latter is valid for a quantitative description for any value of the field parameter p=[U0/(kBT)]1/2. For p≫1 the spectrum comprises one narrow absorption band, while the Debye relaxation region is markedly shifted to low frequencies. It is shown that in the long lifetime τ limit there exists a minimum absorption band Δν0(p). The quantity Δν0 becomes small for values of parameter p≫1. The dielectric relaxation in ice 1 is discussed in relation to this phenomenon.