Vibronic analyses of the lowest singlet-singlet and singlet-triplet band systems of pyridazine

The first singlet-singlet and singlet-triplet band systems of the absorption spectrum of pyridazine vapour are analysed using ab initio and vibronic coupling calculations. The lowest singlet-triplet absorption involves a comparatively unperturbed (pi*,n) B-3(1) state, and contrasts with the highly perturbed singlet-singlet spectrum. The major source of vibronic perturbation in the singlet-singlet absorption is attributed to coupling between near-resonant (pi*, n) (1)A(2) and (n*,n) B-1(1) states, with the former being slightly lower in energy. Many features of this complex and unusual spectrum, and its associated single vibronic level fluorescence spectrum, can be explained using a simple vibronic model, This provides experimental support for recent relaxed CASPT2 and EOM-CCSD calculations, but contrasts with earlier assignments of the spectrum. Theory and experiment suggest that the spacing between the lowest A(2) and B-1 states is larger in the triplet manifold, leading to a simpler spectrum. (C) 2000 Published by Elsevier Science B.V. All rights reserved.