The optical spectrum of the doubly charged molecular nitrogen ion

Emission spectra obtained in the 1550–1650 A region with a 10-m vuv spectrograph are conclusively assigned to the N 2 2+ ion. The 1589-A band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D 1 Σ u + - X 1 Σ g + system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.