Cavity ring-down spectroscopy of the A 2Π3/2–X 2Π3/2 transition of BrO

Cavity ring-down spectroscopy (CRDS) of the BrO A 2Π3/2–X 2Π3/2 electronic transition is shown to be a sensitive laser-based detection method for this radical. Spectra of the A–X (7,0) and (12,0) bands show rotational structure and we report improved spectroscopic constants for the A 2Π3/2 v′=7 and 12 levels and linewidths for the (7,0) and (12,0) transitions derived from fits to the band contours. The measured linewidths of 3.2±0.3 and 4.0±0.4 cm−1 correspond to predissociative lifetimes of the v′=7 and 12 levels of 1.7±0.2 and 1.3±0.2 ps, respectively. From a knowledge of the reaction conditions and decrease in ring-down time on absorption we deduce an upper limit to the concentration of BrO radicals within the CRD chamber of 6×1011 molecules cm−3 and, based on our signal-to-noise ratio, estimate a sensitivity in our experiment of <1010 molecules cm−3.