Extensive ab initio study of the integrated IR intensity in the N2 fundamental collision-induced band

The present paper aims at ab initio investigation of the temperature variations of collision-induced absorption (CIA) intensity in the nitrogen fundamental. Recent measurements (Yu.I. Baranov et al., JMS 233, 160 (2005)) showed that the band integrated CIA intensity increases as soon as the temperature rises above room temperature. This phenomenon can be interpreted in terms of the increasing role played by the induced dipole at a nearly repulsive wall as far as the temperature increases. To check this idea a complete analysis of the CIA intensity based on ab initio calculation is required including evaluations of both the energy and dipole surfaces. We have used the coupled-cluster CCSD(T) technique to calculate the dipole surface from the electric-field perturbed molecular energies (finite-field method). This procedure is complemented with an accurate ab initio potential energy surface and by a semi-classical statistical averaging of the dipole. The calculated integrated intensities are shown to fit the observations within 15%. The increase of the integrated intensity with elevated temperatures is clearly seen.