Collision-induced absorption in the a¹D_g (v'=0) <--- X ³S_g^- (v = 0) transition in O₂-CO₂, O₂-N₂, and O₂-H₂O mixtures

Abstract

When O₂ is perturbed by collisions with other molecules, the weak spin-forbidden magnetic dipole transition a¹D_g <--- X ³S_g^- transition in shows a broad continuum absorption underlying the sharp lines. This collision-induced enhancement absorption plays a role in the Earth's atmosphere and much experimental work has been carried out to measure the binary absorption coefficient with different perturber gases. Recent work on the v' = 0 <--- v = 0 band in O₂-CO₂ mixtures yielded a value for the coefficient that was approximately three times that of earlier measurements on O₂-N₂ mixtures. In the present note, we calculate the absorption theoretically assuming that the long-range quadrupole-induced dipole mechanism is dominant. Using experimental polarizability matrix elements of CO₂ and ab initio results in the literature for the quadrupolar transition matrix element for O₂, we find good agreement for O₂-CO₂ mixtures without any adjustable parameters. The agreement for O₂-N₂ is less good, and because of the much smaller polarizability of N₂ compared with that of CO₂, we suggest that one has to include a short-range component in addition to the long-range one treated here. We also calculate the binary absorption coefficient for O₂-H₂O for which there are no experimental data available and we synthesize the corresponding spectrum for use in atmospheric modeling.

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