The UV-Visible Absorption Cross-Sections and Atmospheric Photolysis Rate of HOI

Abstract

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 the gas phase using laser flash photolysis. The gas phase reaction of OH with I2 was used to produce HOI, and the absorption spectrum of HOI was calibrated relative to the consumption of I2. The HOI spectrum recorded exhibits 2 broad absorption maxima of σ = 3.99 × 10-19 cm2 and σ = 2.85 × 10-19 cm2, centred at 338.4 nm and 404.8 nm respectively. The spectrum is adequately described by a parameterisation consisting of two semi-logarithmic Gaussian distribution functions. The HOI spectrum is more intense than that recorded in previous work of Jenkin, but is in good agreement with more recent work by Bauer et al. The parameterised HOI absorption spectrum recorded in this work was used in a radiative model to calculate the atmospheric photolysis rate (J-value) of HOI. These results indicate that, under most sunlit conditions, HOI has a lifetime with respect to solar photolysis of the order of minutes. Experiments attempting to generate HOI by the reaction 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 desorption of an iodine-containing species in the reaction vessel. This behaviour is rationalised in terms of the disproportionation of HOI to I2O, and an uncalibrated spectrum tentatively attributed to I2O has been recorded.