Ultraviolet-visible absorption cross sections of gaseous HOI and its photolysis at 355 nm

The absorption cross sections of HOI have been obtained at room temperature between 280 and 490 nm. HOI was generated in situ by laser photolytic production of OH in the presence of I-2 and its absorption of UV-visible light measured using both a gated diode array camera and at several wavelengths by a photomultiplier. Calibration was performed relative to I-2 loss. The absorption spectrum consists of two unstructured bands with maxima at 340.4 nm (sigma(max) = (3.85 +/- 0.4) x 10(-19) cm(2)) and 406.3 nm (sigma(max) = (3.30 +/- 0.3) x 10(-19) cm(2)) and is approximated closely by a semilogarithmic two-band Gaussian function. The cross section data were used to calculate an HOI lifetime with respect to photodissociation in the troposphere of between 170 and 100 s for solar zenith angles of 70 degrees and 30 degrees, respectively. The photolysis of HOI was investigated at 355 nm and was found to generate OH radicals with a quantum yield close to unity.