The absorption cross-section and photochemistry of OIO

The absolute absorption cross-section of OIO was measured from 558 to 578 nm by using cavity ring-down spectroscopy to measure the fraction of OIO removed following absorption of a laser pulse of known fluence. This procedure yields σOIO = (1.51 ± 0.18) × 10−17 cm2 at 567.93 nm, which is one of the prominent vibrational band peaks in the OIO spectrum. The recovery of ground-state OIO after a few microseconds indicates that, after excitation from the ground 2B1 to the (first) excited 2B2 state, OIO undergoes rapid internal conversion onto high vibrational levels of the 2B1 state, followed by quenching collisions with the bath gas. A detailed kinetic model is used to show that the OIO yield from the IO self reaction is 0.31 ± 0.10 at 40 Torr and 293 K. The rapid removal of OIO in the reactor is explained by the recombination of atomic I and OIO with a rate constant of (1.1 ± 0.3) × 10−10 cm3 molecule−1 s−1. Ab initio calculations combined with RRKM theory are used to show that this rate constant is consistent with the addition of the I atom to the central I, rather than either of the terminal O atoms. The unexpectedly fast disappearance of I atoms, and the corresponding formation of I2, is explained by iodine oxides such as IO, OIO and I2O3 acting as chaperone molecules.