The absorption cross-section and photochemistry of OIO

Abstract

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 σ OIO = (1.51 ± 0.18) × 10 −17 cm 2 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 2B 1 to the (first) excited 2B 2 state, OIO undergoes rapid internal conversion onto high vibrational levels of the 2B 1 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 cm 3 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 I 2, is explained by iodine oxides such as IO, OIO and I 2O 3 acting as chaperone molecules.