COLOR-CENTER LASER SPECTRUM OF FLUOROBENZENE OBSERVED WITH THE PULSED SUPERSONIC JET EXPANSION TECHNIQUE

Abstract

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 $(a_{1}; \nu_{2}, \nu_{13}, \nu_{20a} b_{1}; \nu_{7b}, \nu_{20b})$ are expected in this region according to the opto-thermal Raman conversion dye laser $spectroscopy^{a}$. So far two parallel bands $(v_{13} v_{20a})$ 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 $results^{b, c}$. The $\nu_{20\mu}$ 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 $-0.5 amu{\AA}^{2}$. 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 $\nu_{11} (\omega = 248 cm^{-1})$ and $\nu_{16b} (498 cm^{-1})$.