We present quantum mechanical calculations of the collision-induced absorption spectra of nitrogen molecules, using ab initio dipole moment and potential energy surfaces. Collision-induced spectra are first calculated using the isotropic interaction approximation. Then, we improve upon these results by considering the full anisotropic interaction potential. We also develop the computationally less expensive coupled-states approximation for calculating collision-induced spectra and validate this approximation by comparing the results to numerically exact close-coupling calculations for low energies. Angular localization of the scattering wave functions due to anisotropic interactions affects the line strength at low energies by two orders of magnitude. The effect of anisotropy decreases at higher energy, which validates the isotropic interaction approximation as a high-temperature approximation for calculating collision-induced spectra. Agreement with experimental data is reasonable in the isotropic interaction approximation, and improves when the full anisotropic potential is considered. Calculated absorption coefficients are tabulated for application in atmospheric modeling.
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The far‐infrared absorption spectra of gaseous nitrogen, and mixtures of nitrogen with the foreign gases argon and neon, have been measured at pressures near 1 atm and at temperatures near 78 and 89 K. Spectra were obtained over the wave number range 20–100 cm−1 using a Fourier transform spectrometer and a multiple reflection absorption cell of 52 m path length. They show a broad continuum associated with the pure rotational collision‐induced S branch of the N2 molecule plus structure attributed to transitions in dimers, not previously observed in this spectral region. In the case of N2–Ar, there are strong similarities with the fundamental vibrational band under similar conditions of pressure and temperature as reported by McKellar [J. Chem. Phys. 88, 4190 (1988)]. The integrated absorption coefficient has been evaluated for the N2–N2 rotational band; at 78 K it equals 3.1×10−31 cm5 sec−1, a factor of 2 greater than typical values at temperatures above 100 K.
