Kinetics of IO Production in the CH2I + O2 Reaction Studied by Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy was used to study the kinetics of formation of IO radicals in the reaction of CH2I + O2 in a flow cell at 52 ± 3 Torr total pressure of N2 diluent and a temperature of 295 K. CH2I was produced by photolysis of CH2I2 at 355 nm and IO probed on the A(2)Π3/2–X(2)Π3/2 (3,0) and (3,1) bands at 435.70 and 448.86 nm, respectively. The rates of formation of IO(v″ = 0) and IO(v″ = 1) were measured as a function of O2 number density using either conventional transient absorption or the simultaneous kinetic and ring-down technique, respectively. IO(v″ = 1) was found to be formed with a significantly larger rate constant, but reached far smaller peak concentrations than IO(v″ = 0). Kinetic modeling supports the conclusion that IO(v″ = 0) is produced both directly and through secondary chemistry, most probably involving the initial formation of the Criegee intermediate CH2OO and subsequent reaction with I atoms, while IO(v″ = 1) is produced exclusively via a direct mechanism. We propose that the reaction mechanism (direct or indirect) depends upon the degree of initial excitation of the photolytically produced CH2I reagent.