Electronic and vibrational structures in the S 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 (⁠$D6h$⁠). 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 $e2g$ fundamental bands and their combination bands with totally symmetric $a1g$ vibrations. This spectral feature is similar to that of benzene although several strong $e2g$ bands are seen in coronene. The band shape (rotational envelope) was significantly different in each $e2g$ 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 $e2g$ vibrational mode, which were obtained by theoretical calculations. The calculated band shapes well reproduced the observed ones, suggesting that the isolated coronene molecule has $D6h$ symmetry.