Configuration interaction effects in molecular spectra. N₂: a case study

A summary is given of configuration interaction effects in diatomic molecules with particular reference to the interpretation of irregularities and anomalies in observed spectra. The method of coupled equations, based on adiabatic states, and the method of matrix diagonalization, based on diabatic states, are discussed in the context of the Born-Oppenheimer approximation and the von Neumann-Wigner non-crossing rule. The recent application of the matrix diagonalization method to the vibronic 1 Σ u + ( b' , c' n ) and 1 Π u ( b , c n , o n ) levels in the far ultraviolet (1000-795 Å) absorption spectrum of N 2 is reviewed and is extended by a J -dependent procedure to the rotational structure of several levels. On the assumption that potential curves and electronic interaction parameters are unchanged on isotopic substitution, the matrix diagonalization procedure has been modified to calculate the vibronic levels in 15 N 2 . A comparison of the results with new high resolution data for 15 N 2 both verifies the validity of the method and testifies to the correctness and accuracy of procedure as applied to the original 14 N 2 data. J -dependent matrix diagonalization has also been carried out for 15 N 2 and sample results are given. Some difficulties have been identified and suggestions are made for possible improvements.