Collision Induced Infrared Absorption of Gaseous Nitrogen at Low Temperatures

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Abstract
The collision induced infrared absorption spectrum of gaseous nitrogen was studied over the temperature range 77–297°K. The integrated absorption coefficient depends quadratically on the gas density and indicates as have previous studies on this and other nonpolar diatomic molecules that the transition inducing mechanism involves a two molecule interaction. A profile analysis of the absorption band at various temperatures assigns the observed features to colliding pairs of freely rotating nitrogen molecules. No evidence for bound state dimers was found although our data do not preclude their existence. Since the integrated absorption coefficient is related to the intermolecular potential, a comparison between the theoretically calculated and experimentally measured values at different temperatures provides a means for studying the potential. It was found that the Lennard‐Jones potential was adequate from 297°K to about 130°K with the traditionally accepted σ and ε/k parameters, but for lower temperatures the agreement with spectroscopic measurements was poor. The discrepancy can be accounted for by realizing that the Lennard—Jones potential is isotropic, but at low temperatures angularly dependent quadrupole interactions become relatively more important and need to be included in the description of the total intermolecular interaction.
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