Knowledge Bank

Understanding of the spectroscopy of molecular oxygen has benefited from many investigations of the absorption and emission spectra from planetary atmospheres, a number of which we have described previously at this symposium. In the process of line-by-line assignment of ``sky spectra'' recorded by astronomers using the Keck I and II telescopes, we have discovered a new $O2$ band system in the 380-450 nm region of the Earth's nightglow spectrum. Sharp isolated lines are assigned to the single Q-branch of $O2(c1\sum u-,v\prime =5-11)$ radiating to $O2(b1\sum g+,v\prime \prime =0-2)$, with intensities that follow the $J\prime (J\prime +1)(2J\prime +1)\exp [-B\prime J\prime (J\prime +1)/kT]$ formula for rotational coupling with $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi$ states suggested by Wilkinson and Mulliken [1] for the corresponding $a\prime -X$ transition in $N2$. Assignment of lines up to $J\prime =32$ allows for improvement of centrifugal distortion constants for the $O2(c)$ state [2,3], while the measured positions of the low J lines agree with those calculated from known term energies [2-4]. The individual lines in the 9-1 c-b band are as intense as any lines in the Herzberg I (A-X) or Chamberlain $(A\prime -a)$ bands at wavelengths longer than 365 nm. Surprisingly, Herzberg II (c-X) band emissions are much weaker than expected from previous studies of the terrestrial [5,6] and venusian nightglows [7,8].