The 1.27‐μm O2 continuum absorption in O2/CO2 mixtures

Abstract

The collision‐induced, near‐infrared O2 continuum band overlapping the weak a1Δg−X3Σg−, v = 0−0, 1.27 μm discrete band of O2 has been investigated in O2/CO2 mixtures at room temperature (T = 296 K) for total densities from 1.8 to 9.3 times that of an ideal gas under standard conditions (T = 273.15 K and P = 101.325 kPa), i.e., from 1.8 to 9.3 amagats. Absorption spectra were recorded at 0.5 cm−1 resolution using a Fourier transform spectrometer and an 84‐m pathlength. A least squares analysis of the integrated band strength, Stotal = SO2ρO2 + SO2 − O2 ρO22 + SO2 − CO2ρO2ρCO2, as a function of the carbon dioxide density, ρCO2, and the oxygen density, ρ02, yields SO2‐Co2 = 2.953(32) × 10−43 cm−2 (molecule/cm3)−2 (i.e., 2.132(23) × 10−4 cm−2 amagat−2). The SO2‐CO2 coefficient is ∼3 times greater than the corresponding SO2‐N2 coefficient determined from studies of O2/N2 mixtures, illustrating the efficiency of large electric multipolar moments in inducing continuum absorption in the 1.27‐μm band of O2. The results support the calculations by Brown and Tipping [2000], which demonstrate the importance of water, with its large electric dipole moment, in enhancing the collision‐induced absorption bands of O2 and N2 in the atmosphere. We suggest that the apparent inability of radiative transfer models to accurately account for the increased atmospheric absorption present when water vapor levels increase may be due in part to the neglect of the intensity enhancement of a number of continuum bands and of the far wings of discrete bands by water molecule collisions.