Structure of pyridazine in the S1 state:: Experiment and theory

The molecular structure of pyridozine in the first electronically excited state (S-1) is deduced from the combined use of resonance-enhanced two-photon ionization and mass-analyzed threshold ionization spectroscopic methods. The equation-of-motion coupled-cluster single and double (EOM-CCSD) calculation gives the distorted planar geometry for the most stable structure of the S-1 pyridazine. The symmetry constraint of C-2v is relaxed to that of C-5, and consequently many in-plane vibrational modes are found to be optically active in both S-1-S-0 and D-0-S-1 excitation spectra, being appropriately assigned from the comparison of their frequencies with ob initio values. This indicates that the S-1-S-0 excitation is partially localized, and provides an alternative explanation for the long-standing spectroscopic puzzle in S-1 pyridazine.