Assessing the contribution of the lowest triplet state to the near-UV absorption spectrum of HOCl

The near-UV absorption spectrum of HOCl is examined using laser induced fluorescence to monitor OH fragments resulting from photodissociation. The measured variation in OH photofragment yields along with an analysis of their Doppler line shapes reveals the presence of a weak absorption feature centered at 380 nm arising from excitation to the lowest excited triplet electronic state of HOCl. Although the peak absorption cross section for the band is modest, sigma(380 nm) = 4 x 10(-21) cm(2), its long-wavelength tail extends down to 480 nm. Present estimates suggest that as a result of absorption by the triplet state, the instantaneous photochemical lifetime of stratospheric HOCl will be shortened by 10-20% and the average abundance reduced by 11-12% relative to previous estimates.