Structure in the Visible Absorption Bands of Jet-Cooled Phenylperoxy Radicals

The visible absorption bands of the phenylperoxy radical in the gas phase have been investigated using cavity ring-down spectroscopy. Jet-cooling was used to reduce the spectral congestion. Structured spectra spanning the range from 17 500 to 19 000 cm^–1 are reported for the first time. Analyses of these data have been guided by the results from time-dependent density functional calculations. The observed spectrum was found to be dominated by the bands of the B̃²A″–X̃²A″ transition. An analysis of the rotational contour for the origin band yielded a homogeneous line width of 2.2 cm^–1, corresponding to a decay rate of 4.1 × 10¹¹ s^–1. The results provide a rationale for the lack of structure in room temperature spectra that have been previously attributed to phenylperoxy. They also indicate that the lower energy region of the spectrum may show resolvable structure at room temperature. If so, this would provide a more definitive signature for monitoring phenylperoxy in kinetic measurements.