We observed the fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled coronene-h and coronene-d. We analyzed the vibronic structures, assuming a planar and sixfold symmetric molecular structure (D). The S state was identified to be B2u1. The SB2u1←SA1g1 transition is symmetry forbidden, so the 0 band is missing in the fluorescence excitation spectrum. We found a number of vibronic bands that were assigned to the e fundamental bands and their combination bands with totally symmetric a vibrations. This spectral feature is similar to that of benzene although several strong e bands are seen in coronene. The band shape (rotational envelope) was significantly different in each e 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 S state and the normal coordinate of each e 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 symmetry.

Corannulene is a nonplanar aromatic hydrocarbon also known as a buckybowl. Its electronic and vibrational structure has been investigated by analyzing its fluorescence excitation spectrum and dispersed fluorescence spectrum in a supersonic jet. Its spectral features are in keeping with the expectation, confirmed by some previous results, that it has fivefold or C5v symmetry. The observed prominent vibronic bands in the S1 ← S0 transition have been assigned to e1 and e2 bands on the basis of theoretical calculations so that the S1 state was assigned to 1E2. The symmetry adapted cluster configuration interaction calculation supports this assignment of the S1 electronic state, although the time-dependent density functional theory calculation suggests that the S1 state is 1A2. It has also been shown that the normal coordinates for strong vibronic bands mainly include out-of-plane vibrational motion. The rotational envelopes are well explained by taking account of the Coriolis interaction between the degenerate vibrational and rotational levels. The mechanism of bowl-to-bowl inversion is also discussed with the results of theoretical calculations regarding the barrier to inversion and metastable conformation.

CH3 internal rotation is one of the typical large amplitude motions in polyatomic molecules, the spectral analysis and theoretical calculations of which, were developed by Li-Hong Xu and Jon Hougen. We observed a Doppler-free high-resolution and high-precision spectrum of 9-methylanthracene (9MA) by using the collimated supersonic jet and optical frequency comb techniques. The potential energy curve of CH3 internal rotation is expressed by a six-fold symmetric sinusoidal function. It was previously shown that the barrier height (V6) of 9MA-d12 was considerably smaller than that of 9MA-h12 [M. Baba, et al., J. Phys. Chem. A 113, 2366 (2009)]. We performed ab initio theoretical calculations of the multicomponent molecular orbital method. The barrier reduction by deuterium substitution was partly attributed to the difference between the wave functions of H and D atomic nuclei.

Bucky bowl is the molecule of nonplanar polycyclic aromatic hydrocarbons. We analyzed the vibronic sructure in the S 1 ← S 0 fluorescence excitation spectra of jet-cooled sumanene and corannulene. The spectrum is congested with a large number of vibronic bands, which are mostly assigned to out-of-plane vibrational modes. The S 1 state of corannulene is identified to 1 E 2 by the normal mode analysis, which is consistent with the result of SAC-CI calculation. The excitation energy of 1 A 2 state was lower than that of the 1 E 2 state by the TD-DFT method. The isolated corannulene molecule is considered to be a normal pentagon with considerable out-of-plane distortion (C 5v ).We observed the IR spectrum of corannulene in solid para-H 2 , which also indicates that the moelcule has a structure with five-fold symmetry in the S 0 state. We found the IR bands originated from protonated corannulene molecules, which are produced by the chemical reaction with a proton.

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