Infrared Spectra of Liquid and Solid Carbon Monoxide

Abstract

A low-temperature cell suitable for the study of the infrared spectra of pure liquids in the fundamental region is described. Selection rules are derived based on the assumption that diatomic molecules in the liquid state can undergo hindered rotation. The infrared absorption band shape is predicted to resemble the P- and R-branch envelope of the gas phase molecule in the wings, but to have an intense Q branch at the band center. The observed spectra of carbon monoxide as a liquid is in reasonable agreement with these selection rules. It is concluded that a considerable fraction of molecules in the liquid state can undergo nearly free rotation. The spectrum of carbon monoxide just below its freezing point suggests, however, that the rotatory motion of the molecules in the solid can best be described as librational.