$1.27 \mu m O_{2}$ CONTINUUM ABSORPTION IN $O_{2}/CO_{2}$ MIXTURES

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 Title: $1.27 \mu m O_{2}$ CONTINUUM ABSORPTION IN $O_{2}/CO_{2}$ MIXTURES Creators: Fraser, G. T.; Lafferty, W. J. Issue Date: 2001 Publisher: Ohio State University Abstract: The collision-induced, near-infrared $O_{2}$ continuum band overlapping the weak $a^{1} \Delta_{g} - X^{3}\Sigma^{-}_{g}$, $\nu = 0 - 0$, $1.27 \mu m$ discrete band of $O_{2}$ has been investigated in $O_{2}/CO_{2}$ 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, $S_{total} = S_{O2}\rho_{O2} + S_{O2-O2}{\rho_{O2}}^{2} + S_{O2-CO2}\rho_{O2}\rho_{CO2}$, as a function of the carbon dioxide density, $\rho CO_{2}$, and the oxygen density, $\rho o_{2}$, yields $S_{O2-co_{2}} = 2.95(40) \times 10^{-43} cm^{-2}(molecule/cm^{3})^{-2}$ [i.e., $2.13(29) \times 10^{-4} cm^{-2} amagat^{-2}$]. The $S_{O2-CO2}$ coefficient is approximately three times greater than the corresponding $S_{O2-N2}$ coefficient determined from studies of $O_{2}/N_{2}$ mixtures, illustrating the efficiency of large electric multipolar moments in inducing continuum absorption in the $1.27 \mu m$ band of $O_{2}$. A similar large enhancement of the $O_{2}$ continuum absorption by $CO_{2}$ is observed for the $\nu = 1 - 0$, $O_{2}$ 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 $O_{2}$ and $N_{2}$ 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. Description: Author Institution: National Institute of Standards and Technology; Optical Technology Division, National Institute of Standards and Technology URI: http://hdl.handle.net/1811/20039 Other Identifiers: 2001-ME-10