Existing laboratory measurements of the far-infrared collision-induced spectra of gaseous nitrogen at temperatures from 124 to 300 K are analyzed on the basis of quantum line shapes computed from a suitable, advanced isotropic potential and multipole-induced dipole functions. The input is chosen to represent most closely the measurements at all temperatures and over the full range of frequencies. Simple analytical expressions are specified which represent the spectral profiles closely. It is thus possible to reproduce the collision-induced absorption spectra of nitrogen effortlessly in seconds at temperatures from 50 to 300 K on small computers, even in the far wings which never have been modeled from a quantum formalism before. The work thus gives new and reliable spectral intensities and their temperature dependence for a detailed analysis of the Voyager IRIS spectra of Titan's atmosphere.
The collision induced spectrum of N2 has been measured in the gas phase at T=300 °K and T=124 °K and in the liquid phase at T=66 °K. The measurements at room temperature have been extended down to 8 cm−1. It has thus been possible to observe the translational component. From the analysis of the band shape the single rotational profile has been derived. The integrated absorption coefficients in the gas phase and the moment analysis of the spectrum are in agreement with the quadrupolar induction mechanism. The presence of cancellation effects has been deduced from the intensity of the spectrum measured in the liquid phase. The absorption coefficient was also obtained from molecular dynamics calculations and compared with the experimental one. Because of the correspondence of their shape we have concluded that also in liquid phase the spectrum is essentially due to the quadrupolar induction mechanism.
The collision-induced absorption (CIA) spectrum for nitrogen has been measured in the spectral region below 360 cm−1 at 126, 149, 179, and 212 K. The measurements have been obtained using Fourier transform infrared (FTIR) techniques, a far infrared (FIR) laser system operating at 84.2 and 15.1 cm−1, and microwave cavity techniques. The experimental line shapes have been compared with the theoretical predictions of Joslin, based on Mori theory, and of Joslin and Gray, based on information theory alone. The data have been used to determine the quadrupole moment employing various intermolecular potentials. One Lennard–Jones potential has resulted in a quadrupole moment of 1.51 B, the value that was used in generating the theoretical line shapes. These results, when combined with our forthcoming measurements on nitrogen mixed with methane and argon, may be helpful in determining the role of CIA in calculating the opacity of some planetary atmospheres.