Electronic and vibrational structures in the S₀ and S₁ states of coronene

We observed the fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled coronene-h₁₂ and coronene-d₁₂. We analyzed the vibronic structures, assuming a planar and sixfold symmetric molecular structure (D_6h). The S₁ state was identified to be B2u1. The S₁B2u1←S₀A1g1 transition is symmetry forbidden, so the 0₀⁰ band is missing in the fluorescence excitation spectrum. We found a number of vibronic bands that were assigned to the e_2g fundamental bands and their combination bands with totally symmetric a_1g vibrations. This spectral feature is similar to that of benzene although several strong e_2g bands are seen in coronene. The band shape (rotational envelope) was significantly different in each e_2g mode. It was shown that degenerate rotational levels were shifted and split by the Coriolis interaction. We calculated the Coriolis parameter using the molecular structure in the S₁ state and the normal coordinate of each e_2g vibrational mode, which were obtained by theoretical calculations. The calculated band shapes well reproduced the observed ones, suggesting that the isolated coronene molecule has D_6h symmetry.