Excitation of the green line in the night airglow

In directions away from the Milky Way, most of the visible light from the night sky comes from a glowing layer of gas at an altitude of ∼97 km. About 20% of this light is concentrated in a sharp green line at 557.7 nm which is a prominant feature of the visible portion of the night airglow. This emission comes from an excited oxygen atom (O( 1 S)), but the mechanism by which this state is populated has been the subject of controversy ever since the emission was first identified in 1925 1 . We present here new laboratory evidence concerning the processes governing the 557.7-nm emission of O( 1 S).