Structure of pyridazine in the S1 state: experiment and theory.

The molecular structure of pyridazine 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(s), 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 ab 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.