Absolute Ultraviolet Absorption Spectrum of a Criegee Intermediate CH₂OO

Abstract

We present the time-resolved UV absorption spectrum of the B̃ (1A′) ← X̃ (1A′) electronic transition of formaldehyde oxide, CH2OO, produced by the reaction of CH2I radicals with O2. 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 σabs = (3.6 ± 0.9) × 10–17 cm2. The difference between the absorption and action spectra likely arises from excitation to long-lived B̃ (1A′) vibrational states that relax to lower electronic states by fluorescence or nonradiative processes, rather than by photodissociation.

The UV absorption spectrum of the simplest Criegee intermediate CH2OO

“…Fig. 4 compares our results with those of Sheps 7 and Beames et al 8 The scaled spectrum of Beames et al 8 is weaker at l Z 360 nm. The temperature effect may be one possible reason for this difference.…”

“…Previous UV absorption 7 and action spectra 8 of CH 2 OO exhibit significant differences. Beames et al 8 measured the laser depletion of CH 2 OO in a similar molecular beam and obtained an action spectrum of CH 2 OO.…”

“…Sheps 7 used a cavity-enhanced technique to measure the UV absorption spectrum of CH 2 OO and observed significant vibrational structures at the long wavelength side of the absorption band. 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.''…”

“…The black and orange lines are scaled to match the absolute cross section at 308.4 nm. The results of Sheps 7 and Beames et al 8 are scaled by a factor of 0.3 for easier comparison. The temperature of the photolysis cell was 295 K. The molecular beams of this work and Beames et al 8 were jet-cooled (estimated rotational temperature B10 K).…”

“…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).…”