Extensiveab initiostudy of the integrated IR intensity in the N₂fundamental collision-induced band

Abstract

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.

H 2 O−N 2 collision-induced absorption band intensity in the region of the N 2 fundamental: ab initio investigation of its temperature dependence and comparison with laboratory data

“…This effect was qualitatively interpreted by Vigasin [10] in terms of an increasing role played upon heating of a gas by induced dipole at extremely close intermolecular separations. Later on, the T -dependence of the integrated CIA in the nitrogen fundamental was nicely reproduced in Lokshtanov et al [11] on the basis of thorough ab initio analysis of the N 2 −N 2 PES and complete dipole surface (DS). It was interesting therefore to examine temperature variations of the integrated CIA in the region of the nitrogen fundamental in the H 2 O−N 2 system, the potential energy and induced dipole properties of which are significantly at variance with respect to N 2 −N 2 system.…”

“…As in our previous work [11], we have calculated the T -dependence of the integrated CIA in the N 2 fundamental in terms of semiclassical approximation. This means that instead of exact quantum summation we have performed classical integration over spatial coordinates of the Boltzmann weighted squared derivative of induced dipole with respect to N−N coordinate.…”

“…It is instructive to compare temperature variations of the CIA intensities in H 2 O−N 2 with those in N 2 −N 2 system borrowed from Lokshtanov et al [11] (figures 7 and 8 and table 2).…”

“…In a sense, we can assert our CCSD(T)-CBS data to be the most reliable to-date estimate for H 2 O−N 2 binary absorption intensity in the region of N 2 fundamental. [11], theoretical curve is slightly corrected after renewed integration procedure, experimental points refer to earlier studies [9,28,29]). Solid line denotes calculated variation; squares, triangles, and circles denote observed variation.…”

“…Following the ideas first formulated in Colpa [27] and then used in Lokshtanov et al [11], the integrated CIA intensity in the N 2 fundamental was calculated using the expression [7] was found by approximate integration from the published graph. Solid curve, CCSD(T); dashed curve, using PES from Tulegenov et al [6]; dot-dashed curve, MP2; filled circles, experimental; open circle, calculated from Brown & Tipping [7].…”