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

Abstract

The present paper aims at ab initio and laboratory evaluation of the N 2 collision-induced absorption band intensity arising from interactions between N 2 and H 2 O molecules at wavelengths of around 4 mm. Quantum chemical calculations were performed in the space of five intermolecular coordinates and varying N−N bond length using Møller-Plesset perturbation and CCSD(T) methods with extrapolation of the electronic energy to the complete basis set. This made it possible to construct the intermolecular potential energy surface and to define the surface of the N−N dipole derivative with respect to internal coordinate. The intensity of the nitrogen fundamental was then calculated as a function of temperature using classical integration. Experimental spectra were recorded with a BOMEM DA3-002 FTIR spectrometer and 2 m base-length multipass White cell. Measurements were conducted at temperatures of 326, 339, 352 and 363 K. The retrieved water-nitrogen continuum significantly deviates from the MT_CKD model because the relatively strong nitrogen absorption induced by H 2 O was not included in this model. Substantial uncertainties in the measurements of the H 2 O−N 2 continuum meant that quantification of any temperature dependence was not possible. The comparison of the integrated N 2 fundamental band intensity with our theoretical estimates shows reasonably good agreement. Theory indicates that the intensity as a function of temperature has a minimum at approximately 500 K.

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

“…most of the contribution at room temperature can be expected to originate from quasi-bound rather than bound complexes [14,29], which may cause an even weaker temperature dependence owing to effective averaging of the intermolecular potential by nearly free rotation of the monomers in such complexes. For the 2.1, 1.6 and 1.2 mm windows, such a relatively small temperature-related change would lie within the average uncertainty of the continuum retrieval in this work, and so cannot be clearly detected; the N 2 −H 2 O CIA in the 4 mm window, according to Baranov et al [27], is expected to have an even weaker temperature dependence within the temperature range investigated here.…”

“…It was suggested in Baranov et al [27] that overtones of the nitrogen fundamental band perturbed by water may also cause more pronounced intensity of the N 2 −H 2 O foreign continuum at higher wavenumbers. The latter, however, according to our results, would require the CIA intensity of the first overtone for the N 2 −H 2 O system to be rather strong; it would have to be weaker than the fundamental N 2 −H 2 O CIA band by not much more than a factor of 5-6 (the ratio of the foreign continuum derived here in the 2300-2400 and 4600-4800 cm −1 spectral regions).…”

“…The strength of the detected absorption is not consistent with the current understanding of the contribution of the far wings of water monomer lines. In most of the 4 mm windows, the water vapour foreign continuum can be reasonably well explained by N 2 −H 2 O BA arising from a collision-induced N 2 fundamental band [25,27]. In the 2.1 and 1.6 mm windows, overtones of the N 2 CIA band and interaction-modified transitions in the H 2 O molecule, including those due to H 2 O−N 2 and H 2 O−O 2 complexes, might be the dominant cause of the H 2 O foreign continuum, but further theoretical work is needed to establish whether or not this is the case.…”

Water in the gas phase

“…Leforestier [10] achieved good agreement between observations and his calculations, when the monomers were treated as being rigid but with the rotational constants of the monomer depending on the dimer geometry. Baranov et al [11] consider H 2 O-N 2 collisioninduced absorption at wavelengths near 4 mm, and find good agreement between laboratory measurements and ab initio theory.…”

Rovibrational analysis of the water bending vibration in the mid-infrared spectrum of atmospherically significant N2–H2O complex

“…There has consequently been increased interest in advancing theoretical studies of N 2 -H 2 O [3,4], particularly with respect to determination of virial coefficients. N 2 collision-induced absorption band intensity arising from interactions between N 2 and H 2 O molecules at wavelengths around 4 m has also been investigated using the ab initio CCSD(T) method complemented by the CBS procedure and FTIR (Fourier transform infrared) laboratory studies [4]. Absorptions due to pure rotational transitions in H 2 O occur in the far-IR (<500 cm −1 ) and are now explicitly accounted for in climate (radiative transfer) models [5].…”