The Fokkerndash;Planckndash;Langevin model for rotational Brownian motion. II. Comparison with the extended rotational diffusion model and with observed infrared and Raman band shapes of linear and spherical molecules in fluids

摘要

A detailed comparison of two extensions of the Debye model for molecular reorientation in liquids, Gordonrsquo;s extendedJhyphen;diffusion model and the rotational Fokkerndash;Planckndash;Langevin model, is presented. It is shown that the two models, although they represent very different physical pictures of rotational dynamics in fluids, lead to remarkably similar reorientational correlation functions, memory functions, correlation times and spectral densities. Only for values of the angular momentum correlation time appropriate to low density fluids do the two models give significantly different results, but the validity of the FPL model is questionable in this region. The results of an infrared study of the ngr;3band of N2O in N2O/O2along the coexistence curve from 105ndash;153 K, and in N2O/N2along the 80 bar isobaric line from 100ndash;300 K are presented. The reorientational correlation functions from these measurements, and from earlier infrared studies of CO/N2and Raman studies of CF4and N2O liquids are compared with correlation functions computed with the EDJ and FPL models. Neither the EDJ nor the FPL model gave a substantially better description of the spectral band shape data.