THEORY OF COLLISION-INDUCED ABSORPTION FOR ELECTRONIC TRANSITIONS IN THE ATMOSPHERICALLY RELEVANT O2−O2 AND O2−N2 PAIRS.

Collision-induced absorption of O$_2-$O$_2$ and O$_2-$N$_2$ pairs is observed in remote sensing of the Earth's atmosphere,_x000d_ and absorption by O$_2-$O$_2$ pairs has been put forward as a biomarker to be observed in exoplanetary transit spectra._x000d_ The relevant electronic transitions, $X~{}^3Sigma_g^- rightarrow a~{}^1Delta_g$ and $X~{}^3Sigma_g^- rightarrow b~{}^1Sigma_g^+$, are electric-dipole forbidden by both spin and spatial selection rules,_x000d_ such that collision-induced absorption represents an important contribution to the absorption._x000d_ _x000d_ We present an emph{ab initio} study of collision-induced absorption for these electronic transitions using quantum-mechanical scattering calculations._x000d_ Two mechanisms for breaking the spin-symmetry are taken into account:_x000d_ intramolecular spin-orbit coupling and intermolecular exchange interactions._x000d_ We find and explain qualitative differences in the line shape and temperature dependence of the absorption due to these mechanisms._x000d_ The contributions of these mechanisms furthermore explain qualitative differences between the atmospherically relevant O$_2-$O$_2$ and O$_2-$N$_2$ systems._x000d_ Reasonable agreement with experimental data is obtained for various near-infrared transitions in both systems._x000d