Absolute Ultraviolet Absorption Spectrum of a Criegee Intermediate CH2OO

We present the time-resolved UV absorption spectrum of the (B) over tilde ((1)A') <- (X) over tilde ((1)A') electronic transition of formaldehyde oxide, CH2OO, produced by the reaction of CH2I radicals with O-2. In contrast to its UV photodissociation action spectrum, the absorption spectrum of formaldehyde oxide extends to longer wavelengths and exhibits resolved vibrational structure on its low-energy side. Chemical kinetics measurements of its reactivity establish the identity of the absorbing species as CH2OO. Separate measurements of the initial CH2I radical concentration allow a determination of the absolute absorption cross section of CH2OO, with the value at the peak of the absorption band, 355 nm, of sigma(abs) = (3.6 +/- 0.9) x 10(-17) cm(2). The difference between the absorption and action spectra likely arises from excitation to long-lived B ((1)A') vibrational states that relax to lower electronic states by fluorescence or nonradiative processes, rather than by photodissociation.