Collision-induced absorption in the a1Δg(v′=0) ← X3Σg- (v=0) transition in O2-CO2, O2-N2, and O2-H2O mixtures

When O-2 is perturbed by collisions with other molecules, the weak spin-forbidden magnetic dipole transition a(1)Delta (g)- X(3)Sigma (-)(x) 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 upsilon'= 0 <-- v = 0 band in O-2-CO2 Mixtures yielded a value for the coefficient that was approximately three times that of earlier measurements on O-2-N-2, 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 O-2, we find good agreement for O-2-CO2 mixtures without any adjustable parameters. The agreement for O-2-N-2 is less good, and because of the much smaller polarizability of N-2 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 O-2-H2O, for which no experimental data are available, and we synthesize the corresponding spectrum for use in atmospheric modeling. (C) 2001 Academic Press.