Orientational disorder in plastic molecular crystals - III. — Infrared and Raman spectroscopy of internal modes

Abstract

The formalism of symmetry adapted functions for molecular orientations introduced previously [5, 6] is here applied to the analysis of the Raman and infrared spectroscopy of internal modes in orientationally disordered molecular crystals. In some favorable cases, the contribution to the Raman and infrared lineshapes arising from the rotational dynamics of the molecules can be disentangled from other contributions arising from the vibrational lifetime or various coupling effects. Here, the rotational lineshapes are analysed in terms of independent, symmetry adapted rotational self-correlation functions. Furthermore, it is shown that the integrated intensity of internal modes provides a measurement of the first symmetry independent coefficients in the development of the orientational probability density function.