Second order infrared absorption spectrum of carbon dioxide in the condensed phase

Solid carbon dioxide crystallizes in the space group [Formula: see text] with four molecules in the primitive unit cell. In the infrared absorption spectra of solid CO2 Mannik and Allin have observed extensive second order features involving combinations of the intermolecular vibrations with the components of the Fermi doublet (ν1, 2ν2) at 88 and 150 K. In the present investigation an interpretation of these second order infrared absorption spectra is carried out employing the rigid ion model of Powell et al. in the framework of external mode formalism and Stephen's theory of the two phonon infrared absorption. Reasonable agreement is observed in the phonon branches along the symmetry directions in our rigid molecule approximation and also in the observed infrared absorption spectra.