The Herzberg bands of O ₂ in an oxygen afterglow and in the night-sky spectrum

An afterglow of flowing oxygen in an uncondensed discharge at around 2 mm pressure shows the $^{3}\sum_{u}^{+}-^{3}\sum_{g}^{-}$ system of O$_{2}$ in emission. Measurements and photographs are presented. The values of v$^{\prime}$ assigned by Herzberg from the absorption spectrum require raising one unit. The afterglow bands are compared with the night-sky spectrum; it is concluded that the Herzberg bands are present in the night sky, but that a few bands previously attributed to the system do not belong to it. The intensity distribution of the system is discussed. The bands are probably excited by recombination of oxygen atoms. The afterglow spectrum also shows the green auroral line and the 'atmospheric' band of O$_{2}$ at 7593 angstrom. A few weak bands around the blue region may be due to a new intercombination system of O$_{2}$, $^{3}\sum_{u}^{+}-^{1}\sum_{g}^{-}$. Diffuse bands in the air afterglow appear to be due to NO$_{2}$. The air afterglow may be responsible for some of the night-sky continuum.