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G. T. FRASER AND W. J. LAFFERTY, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA.
The collision-induced, near-infrared O2 continuum band overlapping the weak a1
g - X 3
g- , 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 = SO2
O2+ SO2-O2
O22 + SO2-CO2
O2
CO2, as a function of the carbon dioxide density,
CO_2, and the oxygen density,
O_2, yields SO_2-CO_2 = 2.95(40) × 10-43 cm-2(molecule/cm3)-2 [i.e., 2.13(29) × 10-4 cm-2 amagat-2]. The SO2-CO2 coefficient is approximately three 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. A similar large enhancement of the O2 continuum absorption by CO2 is observed for the v = 1 - 0, O2 vibrational fundamental. The results support the calculations by Brown and Tipping, 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-vapor collisions.