New O II 6640-Å laser line

A prominent red laser line has been observed in oxygen and air at a wavelength of 6640.2 ± 1 Å. This line corresponds to the
3p^{2}S\min{1/2}\max{0}-3s^{2}P_{1/2}
at 6640.90 Å of the O II spectrum. It is the strongest member of the observed emission lines close to the laser line.