An Interpretation of the Infrared Spectrum of Carbon Monoxide in a Solid Xenon Matrix

Abstract

The infrared absorption band centered at 2133.1 cm−1 of carbon monoxide isolated in solid xenon at 20 °K has been interpreted as due to hindered rotation of the carbon monoxide molecule. The intermolecular interaction, comprising contributions mainly due to dispersion and repulsion, between carbon monoxide and the matrix produces an orientational effect. The rotational motion of the carbon monoxide molecule in the intermolecular field is treated quantum-mechanically so as to account for the main spectral features observed experimentally. From the calculations it is shown that a fraction of CO molecules in the matrix may undergo nearly free rotation to give a broad feature in the infrared band shape resembling the P- and R-branch envelope of the gaseous molecule in the wings. A large fraction of the CO molecules, those with energies less than the potential barrier height, execute librational motion, and thus are responsible for the intense Q branch at the band center.In a smaller matrix cavity, e.g., carbon monoxide in an argon or a krypton crystal, the "over-distortion" of the site would inhibit rotation altogether.