Pressure-Dependent I-Atom Yield in the Reaction of CH2I with O2 Shows a Remarkable Apparent Third-Body Efficiency for O2

The formation of I atom and Criegee intermediate (CH2OO) in the reaction of CH2I with O-2 has potential relevance for aerosol and organic acid production in the marine boundary layer. We report measurements of the absolute yield of I atom as a function of pressure for N-2, He, and O-2 buffer at 298 K. Although the overall rate coefficient is pressure-independent, the I-atom yield, correlated with CH2OO, decreases with total pressure, presumably because of increased stabilization of CH2IOO. The extrapolated yield of the I + Criegee channel under tropospheric conditions is small but nonzero, similar to 0.04. The zero-pressure limiting 1-atom yield is unity, within experimental error, implying negligible branching to IO + CH2O. The apparent collision efficiency of O-2 in stabilizing CH2IOO is a remarkable factor of 13 larger than that of N-2, which suggests unusually strong interaction or possible reaction between the chemically activated CH2IOO# and O-2.