| Popis: |
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 (π ∗ ,n) 3 B 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 (π ∗ ,n) 1 A 2 and (π ∗ ,n) 1 B 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. |
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