Nonadiabatic representations of the 1Σ+u and 1Πu states of the N2 molecule

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Abstract
The vibronic bands in the dipole‐allowed absorptionspectrum of N2 associated with the lowest three electronic 1Σ+ u and the lowest three electronic 1Π u states are represented in a basis of electronically coupled diabatic states as well as in the basis of nuclear‐momentum coupled adiabatic states. Parameters defining the diabatic states and their electronic coupling energies are first evaluated by fitting the eigenvalues of a vibronic interaction matrix to the observations. The coupled‐oscillator equations are then solved directly by Johnson’s numerical integration method and the diabatic representation is redetermined via the matrix method and coupled equations iteratively. The fit of the experimental vibronic terms, B values, and absorption intensities achieved with R‐independent electronic coupling energies in a diabatic basis of valence and Rydberg‐type states (b′+c′+e′)1Σ+ u and (b+c+o)1Π u is satisfactory. Comparison with the corresponding adiabatic representation shows that the nonadiabatic perturbations are larger in that basis than in the diabatic one. The vibronic intensity distributions observed in the absorptionspectrum show numerous interesting examples of intensity envelopes over discrete vibronic progressions with Fano‐type resonance profiles as well as with distinctly non‐Fano‐type profiles which can be attributed to variations of the relevant coupling terms over the widths of the vibronic resonances.