Cavity ring-down spectroscopy of the A2Π3/2-X2Π3/2 transition of BrO

Cavity ring-down spectroscopy (CRDS) of the BrO A(2)II(3/2)-(XII3/2)-I-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(2)II(3/2) upsilon(') = 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 upsilon' = 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 x 10(11) molecules cm(-3) and, based on our signal-to-noise ratio, estimate a sensitivity in our experiment of < 10(10) molecules cm(-3). (C) 1998 Elsevier Science B.V.