Recombination of iodine atoms by flash photolysis over a wide temperature range. VIII. I2 in O2

Abstract

Flash photolysis of I2 in a large excess of O2 was studied between 300 and 1173 K. Between 573 and 1173 K the rate of atomic recombination [Formula: see text]was measured without interference of side reactions. Its rate constant was found to be [Formula: see text]. Between 573 and 873 K there is an induction period lasting several hundred microseconds during which only a small net change in the iodine molecule concentration takes place. This induction period is consistent with the reaction [Formula: see text] Beow 573 K, the overall recombination rate constant has a temperature dependent negative temperature coefficient and is enhanced when the flash includes radiation in the neighborhood of 200 nm. Evidently this occurs because the flash photolysis of O2 creates new efficient third bodies, such as O3 and O2 (1Δg), which accelerate the recombination reaction either directly or via an IO intermediate. The mechanism for the formation of IO during the flash photolysis of I2 in an excess of O2 was studied. It was shown that the reactions O(3P) or [Formula: see text], where O3* refers to an electronically or vibrationally excited state of O3, account for the formation of IO under our experimental conditions. IO may also possibly be formed via the reaction [Formula: see text]. On the other hand, reactions between excited or ground state I atoms and O2 suggested by earlier workers do not occur. Thus, all experimental complications observed are traceable to the presence of O2 photolysis products and to the reaction of I(2P1/2) with O2(3Σg−).