Ultraviolet Spectrum and Photochemistry of the Simplest Criegee Intermediate CH2OO

Ozonolysis of alkenes in the troposphere produces Criegee intermediates, which have eluded detection in the gas phase until very recently. This laboratory has synthesized the simplest Criegee intermediate within a quartz capillary tube affixed to a pulsed valve to cool and isolate CH2OO in a supersonic expansion. UV excitation resonant with the B (1)A' <- X (1)A' transition depletes the ground-state population of CH2OO, which is detected by single-photon ionization at 118 nm. The large UV-induced depletion (approaching 100%) near the peak of the profile at 335 nm is indicative of rapid dissociation, consistent with the repulsive B (1)A' potential along the O-O coordinate computed theoretically. The experimental spectrum is in very good accord with the absorption spectrum calculated using the one-dimensional reflection principle. The B <- X spectrum is combined with the solar actinic flux to estimate an atmospheric lifetime for CH2OO at midday on the order of similar to 1 s with respect to photodissociation.