15min:

M. ISHIGURO, K. HARADA, K. TANAKA AND T. TANAKA, Department of Chemistry, Faculty of Science, Kyushu University 33, Higashiku, Fukuoka 812-8581, Japan..

The infrared spectrum of fluorobenzene in the CH-stretch region (3040-3120~cm^-1) has been observed with a color-center laser spectrometer combined with a pulsed supersonic jet expandion technique. Due to the rotational as well as cooling in the jet, well resolved rovibrational lines were recorded. At least 5 fundamental bands (a₁; ₂,₁₃,_20a b₁; _7b,_20b) are expected in this region according to the opto-thermal Raman conversion dye laser spectroscopy . So far two parallel bands (₁₃,_20a) were analyzed to confirm the vibrational assignments given by the low-resolution spectroscopy.

The rotational constants and band origins were derived from the observed spectra assuming those for ground state to the microwave results ^, . The _20a band were found to be overlapped additionally with the two vibrational bands presumably caused by higher order Fermi interaction, whose inertia defects are small negative about -0.5 amuŲ. Their vibrational assignments are obscure yet, but the negative inatria defects imply that they include two or four quanta of low frequency out-of-plane vibrational modes such as ₁₁(\omega=248~cm^-1) and _16b(498~cm^-1).