Breaking down a Criegee intermediate: Ozone's damaging role in the upper atmosphere is well known, but ozone is also quite active closer down to where we live. In particular, ozone's run-ins with airborne unsaturated hydrocarbons, from natural or anthropogenic sources, produce even more-reactive OH radicals. Liu et al. used vibrational spectroscopy to study how OH emerges from a so-called Criegee intermediate formed when ozone attacks 2-butene. The results suggest that OH production is easier than current theory predicts. Science , this issue p. 1596

Ozonolysis of alkenes in the troposphere proceeds through a Criegee intermediate, or carbonyl oxide, which has only recently been detected in the gas phase. The present study focuses on the production of an alkyl-substituted Criegee intermediate, CH₃CHOO, in a pulsed supersonic expansion, and then utilizes VUV photoionization at 118 nm and UV-induced depletion of the m/z = 60 signal to probe the B ¹A^′ ← X ¹A^′ transition. The UV-induced depletion approaches 100% near the peak of the profile at 320 nm, indicating rapid dynamics in the B state, and corresponds to a peak absorption cross section of ∼5 × 10⁻¹⁷ cm² molecule⁻¹. The electronic spectrum for CH₃CHOO is similar to that reported recently for CH₂OO, but shifted 15 nm to shorter wavelength, which will result in a longer tropospheric lifetime for CH₃CHOO with respect to solar photolysis. Complementary electronic structure calculations (EOM-CCSD) are carried out for the B and X potentials of these Criegee intermediates along the O–O coordinate. An intramolecular interaction stabilizes the ground state of the syn-conformer of CH₃CHOO relative to anti-CH₃CHOO, and indicates that the syn-conformer will be the more abundant species in the expansion. The excited B electronic state of syn-CH₃CHOO is also predicted to be destabilized relative to that for anti-CH₃CHOO and CH₂OO, in accord with the shift in the B-X transition observed experimentally. Hydroxyl radicals produced concurrently with the generation of the Criegee intermediates are detected by 1+1^′ resonance enhanced multiphoton ionization. The OH yield observed with CH₃CHOO is 4-fold larger than that from CH₂OO, consistent with prior studies of OH generation from alkene ozonolysis.