The ungerade singlet states of molecular nitrogen observed up to 115 000 cm−1 have traditionally been designated with some 20 letter symbols. It is shown that these levels do not belong to 20 different electronic states but that all of them can be ordered into six vibrational progressions of three valence states: b1Πu (j, b, d, m, p, q), b′ 1Σu+ (b′, g, g′, f, r), d′(1Σu− or 1Δu?), and of three Rydberg states: c1Πu (l, d″), c′ 1Σu+ (p′, r′, k, s′, h, h′), and o1Πu. The new assignments of observed levels to the states b, b′, c, and c′ are identical with those given by Carroll and Collins on the basis of new high-resolution studies of the absorption spectrum. It is shown here, that the irregularities of the vibrational intervals, rotational constants, isotope shifts, and intensity distributions within these progressions can be interpreted quantitatively on the basis of homogeneous interactions between valence and Rydberg states of the same species, especially between the pairs of states b1Πu + c1Πu and b′ 1Σu+ + c′ 1Σu+. Approximate quantitative deperturbations of the vibrational structures of these four electronic states are derived from a new set of deperturbation criteria, and the resultant potential curves, the electron configurations, and the observed predissociations are discussed. The deperturbation results for the b′ and c′ states are tested in more quantitative detail by Lefebvre-Brion in the adjoining paper.

Dressler, K. (1969). The lowest valence and Rydberg states in the dipole-allowed absorption spectrum of nitrogen. A survey of their interactions. Canadian Journal of Physics, 47(5), 547–561.