We observed microwave absorption spectra of some deuterated benzenes and accurately determined the rotational constants of all H/D isotopomers in the ground vibrational state. Using synthetic analysis assuming that all bond angles are 120°, the mean bond lengths were obtained to be r0(C-C) = 1.3971 Å and r0(C-H) = r0(C-D) = 1.0805 Å. It has been concluded that the effect of deuterium substitution on the molecular structure is negligibly small and that the mean bond lengths of C-H and C-D are identical unlike small aliphatic hydrocarbons, in which r0(C-D) is about 5 mÅ shorter than r0(C-H). It is considered that anharmonicity is very small in the C-H stretching vibration of aromatic hydrocarbons.

We investigated the S1 and S2 states of linear and zigzag cata-condensed hydrocarbons on the basis of the results of jet spectroscopy and theoretical calculations. The S1 states of anthracene and tetracene are represented by the HOMO → LUMO configuration (Φ(A)), whereas those of phenanthrene and chrysene are represented by HOMO-1 → LUMO and HOMO → LUMO+1 configurations (Φ(B)). We found that the fluorescence lifetime varied with different vibronic levels in the S1 states of linear cata-condensed hydrocarbons due to the mode-selective internal conversion to the S0 state. This selectivity is likely to be seen in the S1 Φ(A) state of the D(2h) molecule.

High-resolution spectra of the S1←S0 transition in jet-cooled deuterated benzenes were observed using pulse dye amplification of single-mode laser light and mass-selective resonance enhanced multiphoton ionization (REMPI) detection. The vibrational and rotational structures were accurately analyzed for the vibronic levels in the S1 state. The degenerate 6(1) levels of C6H6 or C6D6 are split into 6a(1) and 6b(1) in many of deuterated benzenes. The rigid-rotor rotational constants were assessed and found to be slightly different between 6a and 6b because of different mean molecular structures. Their rotational levels are significantly shifted by Coriolis interactions. It was found that the Coriolis parameter proportionally changed with the number of substituted D atoms.

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.

We observed the fluorescence excitation spectra and mass-selected resonance enhanced multiphoton ionization (REMPI) excitation spectra for the 6(0)(1), 6(0)(1)1(0)(1), and 6(0)(1)1(0)(2) bands of the S1←S0 transition of jet-cooled deuterated benzene and assigned the vibronic bands of C6D6 and C6HD5. The 6(0)(1)1(0)(n) (n = 0, 1, 2) and 0(0)(0) transition energies were found to be dependent only on the number of D atoms (ND), which was reflected by the zero-point energy of each H/D isotopomer. In some isotopomers some bands, such as those of out-of-plane vibrations mixed with 6(1)1(n), make the spectra complex. These included the 6(1)10(2)1(n) level or combination bands with ν12 which are allowed because of reduced molecular symmetry. From the lifetime measurements of each vibronic band, some enhancement of the nonradiative intramolecular vibrational redistribution (IVR) process was observed. It was also found that the threshold excess energy of "channel three" was higher than the 6(1)1(2) levels, which were similar for all the H/D isotopomers. We suggest that the channel three nonradiative process could be caused mainly by in-plane processes such as IVR and internal conversion at the high vibrational levels in the S1 state of benzene, although the out-of-plane vibrations might contribute to some degree.