The UV-visible absorption cross-sections and atmospheric photolysis rate of HOI

The UV-visible absorption cross-sections of HOI have been recorded over the wavelength range 278-494 nm and at 298 K following generation of HOI in th e gas phase using laser flash photolysis. The gas phase reaction of OH with I-2 was used to produce HOI, and the absorption spectrum of HOI was calibr ated relative to the consumption of I-2. The HOI spectrum recorded exhibits 2 broad absorption maxima of sigma = 3.99 x 10(-19) cm(2) and sigma = 2.85 x 10(-19) cm(2), centred at 338.4 nm and 404.8 nm respectively. The spectr um is adequately described by a parameterisation consisting of two semi-log arithmic Gaussian distribution functions. The HOI spectrum is more intense than that recorded in previous work of Jenkin, but is in good agreement wit h more recent work by Bauer et al. The parameterised HOI absorption spectru m recorded in this work was used in a radiative model to calculate the atmo spheric photolysis rate (J-value) of HOI. These results indicate that, unde r most sunlit conditions, HOT has a lifetime with respect to solar photolys is of the order of minutes. Experiments attempting to generate HOI by the r eaction of O atoms with C2H5I led to complex absorption spectra containing a negative contribution to the absorption from the photolytic removal of an unidentified species. In addition, evidence was found for adsorption and d esorption of an iodine-containing species in the reaction vessel. This beha viour is rationalised in terms of the disproportionation of HOI to I2O, and an uncalibrated spectrum tentatively attributed to I2O has been recorded.