DIELECTRIC RESPONSE AND A PHENOMENON OF A NARROW-BAND ABSORPTION FOR A CLASSICAL ROTOR IN A DOUBLE-WELL POTENTIAL

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 = U-0 sin(2) theta. The evolution of wideband dielectric spectra as a function of the potential well depth U-0 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 held parameter p=[U-0/(KBT)](1/2). For p much greater than 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 tau limit there exists a minimum absorption band Delta nu(0)(p). The quantity Delta nu(0) becomes small for values of parameter p much greater than 1. The dielectric relaxation in ice 1 is discussed in relation to this phenomenon. (C) 1995 American Institute of Physics.