A polarized single crystal Raman study of the librational phonons in p-diiodobenzene

Single crystal polarized Raman spectra of p-diiodobenzene in the region from 0 to 300 cm-1 are presented for sample temperatures from 2 to 330 K. Eleven of the twelve predicted gerade librational phonons are unambiguously assigned in terms of their space group symmetries. The temperature dependence of the Raman spectra does not show a soft gerade phonon at the nearly second-order phase transition which occurs at 326 K. A significant anharmonicity for molecular motions above the I-I axis is indicated by the observation of overtones and combinations and broadening of these phonons as the temperature is raised from 2 K. Copyright © 1976 American Institute of Physics.