15min:

MASAAKI BABA, YUKI NOMA, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; SHUNJI KASAHARA, Molecular Photoscience Research Center, Kobe University, Kobe 657-8501, Japan; TAKAYA YAMANAKA, Institute for Molecular Science, Okazaki 444-8585, Japan; JON T. HOUGEN, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA.

We have analysed energy levels of CH₃ torsion in the S₀ and S₁ states of 9-methylanthracene (9MA) by high-resolution fluorescence excitation spectrum in a supersonic jet. 9MA has the G₁₂ molecular symmetry, which is the same as toluene. The sixfold torsional barrier in toluene is very small, and the barrier height has been accurately determined as V₆(S₀) = 4.874 cm^-1 and V₆(S₁) = 26.376 cm^-1 by microwave and ultrahigh-resolution laser spectroscopy. ^, In 9MA, we estimated the barrier height as V₆(S₀) ~ 100 cm^-1 and V₆(S₁) ~ 50 cm^-1, which is remarkably larger than that in toluene. The difference of transition energies between the 0a₁' 0a₁' and 1e'' 1e'' is expected to be about 1 cm^-1. We have observed rotationally resolved ultrahigh-resolution spectrum of 9MA in a collimated molecular beam and confirmed this splitting. All of the observed rotational lines exhibit B-type selection rule, and the direction of transition moment has been shown to be parallel to the C₃ axis of the CH₃ group.