Vibrational correlation-function modeling in the context of the Zwanzig–Mori formalism. Application to liquid ethane

Abstract

The relationship between the memory-function approach to correlation-function modeling and the Mori continued-fraction scheme suggests that unsuspected dynamical couplings may require higher-order procedures to adequately model some correlation functions. The application of first-, second-, and third-order memory-function procedures and their significance is discussed. Some guidelines for the experimentalist are given, and the problem of the truncation of isotropic Raman bands is discussed. The spontaneous Raman-scattering spectrum of the ν3 band of C2H6 has been reexamined for sample temperatures in the range 93–168 K. Both the isotropic and anisotropic spectra have been obtained, and the vibrational-correlation function has been modeled within experimental error. The need to go to third order in the modeling procedure and the values of the second moments of the IVH spectra suggest a vibrational–rotational coupling. The depolarization ratio for the ν3 band was observed to be 0.21.