UV SPECTRUM AND PHOTOCHEMISTRY OF THE SIMPLEST CRIEGEE INTERMEDIATE CH$_2$OO

Abstract

In the troposphere, ozonolysis of ethene as well as other terminal alkenes, such as isoprene, results in formation of the simplest Criegee intermediate, CH$_2$OO, which gives rise to many products of atmospheric significance. In the laboratory, CH$_2$OO is prepared in a quartz capillary tube reactor using a new synthetic route, 204 (2012).} based on 248 nm photolysis of CH$_2$I$_2$ and subsequent reaction with O$_2$. The CH$_2$OO and other products undergo supersonic expansion, and are detected using fixed frequency VUV ionization (118 nm) with mass (m/z=46) and isomer selectivity. Spectroscopic studies of CH$_2$OO are carried out by UV laser excitation on the B $^1$A$^\prime$-X $^1$A$^\prime$ transition prior to photoionization, which results in significant depletion of the ion signal (approaching 100\%) near the peak of a broad absorption profile centered at 335 nm., 20045 (2012).} The large depletion and broad absorption are indicative of rapid dissociation, consistent with the repulsive B $^1$A$^\prime$ state potential in the O-O coordinate that has been computed theoretically. The experimental spectrum is in very good agreement with the absorption spectrum calculated using the one-dimensional reflection principle. The atmospheric lifetime of CH$_2$OO due to solar photolysis at midday is estimated to be $\sim$ 1 s. These findings provide a UV spectral signature of the strong $\pi$$^\ast$-$\pi$ transition associated with the four $\pi$ electrons on the carbonyl oxide group in CH$_2$OO. Extensions of this work to methyl substituted Criegee intermediates reveal similar absorption features arising from the COO $\pi$ -system .