The collision-induced fundamental absorption spectra of N2 and O2 pairs

In a series of three recent papers, we have carried out theoretical studies of the collision-induced fundamental absorption of N2-N2, O2-O2, and N2-O2 and O2-N2 pairs, respectively, where the first molecule in the pair makes the fundamental vibrational transition. In these studies we considered a number of induction mechanisms characterized by their angular symmetries; these included the dominant, long-range isotropic and anisotropic quadrupolar inductions, and the analogous weaker hexadecapolar inductions. In the O2-O2 study, we also investigated the effects of including higher-order induction mechanisms. The necessary molecular matrix elements of the multipole moments and the isotropic and anisotropic polarizability were obtained from the literature. These were varied independently within their expected uncertainties for N2-N2 and O2-O2 in order to obtain the best fits. Using these 'optimal parameters' we then found that there were significant differences between theory and experiment for the mixed pairs N2-O2 and O2-N2. By adding a shortrange component to the dominant quadrupole-induced dipole, we were able to obtain good global fits to all the temperature-dependent integrated intensities and profiles for all four systems. In an attempt to account for the small dips having their minima at the positions of the Raman transitions, we modified the collision-induced line shape to account for intercollisional interference. However, our results were inconsistent with the expected density dependence for this effect. Recently, we considered the effects of metastable-metastable dimer transitions, and by including these we are able to obtain both qualitative and quantitative agreement between theory and experiment for all the systems over a wide range of pressure and temperature