Absorption of dipolar liquids in the far infrared: a sensitive measure of the Mori continued fraction

Abstract

Using a specially developed polarising interferometer for the range 2–200 cm–1, the complete rotational type absorptions of some halogenobenzenes and of tertiary butyl chloride have been obtained in the liquid phase at 296 K. The broad experimental bands have contributions from permanent and induced, temporary dipoles; the absorption due to the latter being negligibly small below about 2 cm–1, but (gradually) becoming predominant at the higher wave numbers thereafter. The auto-correlation function of the permanent dipole unit vector (u) is simulated by a quotient of polynomials obtained by assuming an exponential, Gaussian, or Lorentzian form for the second memory function K1(t). All three theoretical absorptions therefrom reduce to the classical Debye form at low frequencies, but behave very differently in the far infrared, a region particularly sensitive to dynamical events at times shortly after the arbitrary initial t= 0. A fairly realistic result is obtained only from the exponential K1(t), which gives an integrated absorption intensity of about half that observed in the far infrared. The excess is due mainly to collisionally induced absorption, but also in part to the difference between the dynamic internal field and the Maxwell field. K1(0) is evaluated for the asymmetric top, revealing that it is dependent in general on separable torque and centripetal terms.If the theoretical method employed here is to be generally successful, then K1(t) must be simulated more realistically, possibly with the aid of computer molecular dynamics.