Ultraviolet Spectrum and Photochemistry of the Simplest Criegee Intermediate CH₂OO

Abstract

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 CH(2)OO in a supersonic expansion. UV excitation resonant with the B (1)A' ← X (1)A' transition depletes the ground-state population of CH(2)OO, 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 CH(2)OO at midday on the order of ∼1 s with respect to photodissociation.

The UV absorption spectrum of the simplest Criegee intermediate CH2OO

“…Moreover, the absorption spectrum 7 differs significantly from the action spectrum reported by Beames et al 8 Sheps' argument 7 is the following: ''The difference between the absorption and action spectra likely arises from excitation to long-lived B ( 1 A 0 ) vibrational states that relax to lower electronic states by fluorescence or nonradiative processes, rather than by photodissociation.'' However, the measurement of the photoproduct anisotropy 10 indicates that the photodissociation is faster than rotation which is in the picosecond time scale.…”

“…1c, it is clear that the intensity of band A decreases at higher SO 2 concentrations. Because the absorption cross sections of SO 2 , H 2 CO and SO 3 in the 316-450 nm range are much smaller (o1 Â 10 À19 cm 2 ) than those of CH 2 OO (peak cross section 41 Â 10 À17 cm 2 ), 8 the differences between the spectra of Fig. 1c should be mostly due to the absorption of CH 2 OO.…”

“…Without using a laser beam profiler, it is difficult to quantify the actual laser fluence. Beames et al 8 might underestimate their laser fluence and thus overestimate the CH 2 OO cross section. In this work, we utilized a reference molecule to effectively calibrate the laser fluence and to cancel the effect of non-uniform laser spot.…”

“…8 As a result, the OH formation of (R4) + (R5) may contribute significantly to the atmospheric OH concentrations.…”

“…Now it is generally accepted that ozonolysis of alkenes proceeds via Criegee intermediates, highly reactive species postulated in 1949 by Rudolf Criegee. 1,2 In the troposphere, Criegee intermediates are involved in several important atmospheric reactions, 3 including reactions with SO 2 and NO 2 , [4][5][6][7] or can be photolyzed by near UV light, [7][8][9][10] as shown for CH 2 The formation of SO 3 and NO 3 , as in (R2) and (R3), plays an important role in atmospheric chemistry, 11,12 including aerosol and cloud formation. The formation of O( 1 D), as in (R4), will result in OH formation through (R5).…”