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

Abstract

The formation of I atom and Criegee intermediate (CH₂OO) in the reaction of CH₂I with O₂ 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 N₂, He, and O₂ 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, ∼0.04. The zero-pressure limiting I-atom yield is unity, within experimental error, implying negligible branching to IO + CH₂O. The apparent collision efficiency of O₂ in stabilizing CH₂IOO is a remarkable factor of 13 larger than that of N₂, which suggests unusually strong interaction or possible reaction between the chemically activated CH₂IOO^# and O₂