O2–O2 CIA in the gas phase: Cross-section of weak bands, and continuum absorption between 297–500 nm
Henning Finkenzeller, Rainer Volkamer
Abstract
Oxygen–oxygen collision-induced absorption (O2–O2 CIA) of solar radiation heats the atmosphere, affects radiative transfer, and needs to be considered in spectroscopy applications in Earth’s atmosphere. The lack of O2–O2 CIA spectra in the gas phase below 335 nm wavelength is limiting remote sensing applications in this spectral range. This study reports measurements of the O2–O2 CIA cross-section in the gas phase at high signal-to-noise ratio and spectral resolution, sufficient to fully resolve spectral band shapes (0.31–0.42 nm FWHM, full width at half maximum) using Cavity Enhanced Extinction Spectroscopy at atmospheric pressure, and variable temperature (293, 263, and 223 Kelvin). Excellent agreement with literature spectra is observed for selected strong bands. Several heretofore unmeasured weak absorption bands are characterized for the first time in the gas phase under controlled laboratory conditions, i.e., the bands centered at 315 and 328 nm (b1Σg++b1Σg+←X3Σg−+X3Σg−, vibrational quanta change Δνtot=4,3), 420 nm (a1Δg+b1Σg+←X3Σg−+X3Σg−,Δνtot=2), and 495 nm (a1Δg+a1Δg←X3Σg−+X3Σg−,Δνtot=3). The band shape at 344 nm deviates from the available literature, in part owing to the overlap of neighboring bands that give rise to non-zero continuum absorption. The consistency of spectral band shapes of transitions with non-zero Δνtot is used to predict the absorption bands at 301 nm (b1Σg++b1Σg+←X3Σg−+X3Σg−, Δνtot=5), and 397 nm (a1Δg+b1Σg+←X3Σg−+X3Σg−,Δνtot=3). A complete O2–O2 CIA cross section spectrum is presented between 297–500 nm, that is optimized for use in hyperspectral (and other) remote sensing applications.