Theoretical analysis of the collision-induced electronic absorptions in O2-N2 and O2-CO2 pairs

Theoretical calculations are made for the collision-induced electronic absorption in the atmospheric infrared a←X (0,0) and visible b←X (0,0) bands of oxygen in O2-N2 and O2-CO2 pairs. We consider only the long-range quadrupolar induction mechanism and use ab initio values for the quadrupole transition matrix elements that arise from spin-orbit mixing. By fitting the experimental value for the a←X (0,0) absorption in O2-N2, we obtain estimates for the isotropic and anisotropic polarizability electronic transition matrix elements of O2. As a check, we then use these results together with other known data to calculate the corresponding absorption for O2-CO2. We find good agreement, considering experimental error and theoretical approximations. We then estimate the corresponding absorption for the b←X (0,0) absorption in O2-N2. We discuss briefly the observed collision-induced absorption by O2-O2 pairs, and conclude that there must be other induction mechanisms involved for this system. Because of the importance of absorption by O2 in the Earth's atmosphere, we conclude that a systematic study, including both additional measurements and theoretical analyses must be carried out before a satisfactory understanding can be attained.