Collision-Induced Absorption in the a1Δg(v′=0)←X3Σg−(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 a1Δg←X3Σg− 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 than 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 no experimental data are available, and we synthesize the corresponding spectrum for use in atmospheric modeling.