Cavity ring down spectroscopy on C6H5 radical in a pulsed supersonic jet expansion discharge

The formation mechanisms of polycyclic aromatic hydrocarbons (PAH's) are of interest due to the importance of these molecules in the fields of combustion, atmospheric chemistry, interstellar chemistry, and environmental health. The detection and characterization of the highly reactive intermediate species in PAH formation reactions is a key step in the investigation of the formation mechanisms. However, the low concentrations associated with these radicals requires high sensitivity techniques for spectroscopic characterization. This study combines an electrical discharge to produce radicals, a pulsed jet expansion to cool them immediately after production, and cavity ringdown spectroscopy (CRDS) for detection. In our first demonstration of CRDS system, we report the absorption spectrum of the C₆H₅ (phenyl) radical ²B₁-²A₁ transition with a rotational temperature near 10 K. Rotational structure is present in our spectra since only lower energy rotational states are populated, greatly simplifying each vibronic band. Therefore we obtained rotational constants and vibrational frequencies of various vibrational states in the ²B₁ electronic state by contour fits of various vibronic bands. These results show the effectiveness of our system for studying relatively large, reactive intermediates. The data from this and future studies can be combined with theoretical models to describe PAH formation.