Zero-THz spectroscopy of rotator phases

The molecular dynamical evolution from hertzian frequencies to the far infrared (THz) is reported for various kinds of rotator phase environment. The zero to THz frequency spectral profiles are analysed in terms of itinerant libration and developments on this theme. The systems analysed are (i) the plastic rotator phase of t-butyl chloride and solid solutions of this solute in tetramethylsilane; (ii) these results are compared with those from a spectral study in the range up to THz frequencies of t-butyl chloride in vitreous decalin solvent; (iii) single crystalline pentachloronitrobenzene and a pressed disc thereof. This solid is representative of a class of rotator phases where orientational disorder is gradually frozen in at low temperatures, unlike the plastic rotators; (iv) solid solutions of dipolar solute molecules in non-dipolar rotator phase benzene and tetramethylsilane. This is an attempt at evaluating the electrodynamics of dipole-dipole interaction.

The data over the complete range of frequency is rationalized in terms of the itinerant libration mean square torque and theoretical evaluations of the observable sub-THz Debye relaxation times. This is the first attempt at modelling the molecular dynamics of the various kinds of rotator phases over the complete range of frequency at which the kinematics and electrodynamics of the constituent molecules are observable, either as the complex dielectric permittivity or the optical power absorption coefficient. This methodology is named zero-THz spectroscopy. Used properly, it is one of the most incisive methods available for studies of molecular dynamics.