HIGH RESOLUTION INFRARED SPECTROSCOPY OF JET-COOLED PHENYL RADICAL IN THE GAS PHASE

Abstract

Phenyl radical (C$_6$H$_5$) is one of the most important reactive intermediates, as it is formed from the homolytic cleavage of a CH bond in benzene (C$_6$H$_6$), and hence it plays a central role in the combustion of fossil fuels that are typically rich in aromatics. We recently recorded the first high resolution infrared spectra of jet-cooled phenyl radical in the gas phase. This was obtained by direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion of a phenyl halide precursor (C$_6$H$_5$X, $i.e.$~C$_6$H$_5$I and C$_6$H$_5$Br) diluted in a Neon/Helium gas mixture. We observed an A-type band, which arises from a fundamental excitation of the out-of-phase symmetric CH stretch ($\nu_{19}$). The unambiguous assignment of the rotational structure in this band to C$_6$H$_5$ is facilitated by comparing 2-line combination differences with the Fourier transform microwave (FTM) and direct absorption millimeter-wave (mm-wave) measurements of the ground state by McMahon~$et~al.$ \textbf{590}, L61~(2003).}. A least-squares fit to an asymmetric top Hamiltonian of the rotationally-resolved vibrational band is done to determine upper-state rotational constants and a gas-phase band origin ($\nu_0$) of 3071.8904~(10)~cm$^{-1}$. This is in very good agreement with the value of 3071~cm$^{-1}$ for the out-of-phase symmetric CH stretch of phenyl reported by Friderichsen $et~al.$ \textbf{123}, 1977~(2001).} from matrix isolation studies, which indicates a surprisingly small red shift due to the low-temperature argon environment.