15min:

JOSEPH M. BEAMES, FANG LIU, LU LU AND MARSHA I. LESTER, Department of Chemsitry, University of Pennsylvania, Philadelphia, PA 19104-6323.

In the troposphere, ozonolysis of ethene as well as other terminal alkenes, such as isoprene, results in formation of the simplest Criegee intermediate, CH₂OO, which gives rise to many products of atmospheric significance. In the laboratory, CH₂OO is prepared in a quartz capillary tube reactor using a new synthetic route based on 248 nm photolysis of CH₂I₂ and subsequent reaction with O₂. The CH₂OO 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 CH₂OO are carried out by UV laser excitation on the B ¹A'-X ¹A' 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. The large depletion and broad absorption are indicative of rapid dissociation, consistent with the repulsive B ¹A' 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 CH₂OO due to solar photolysis at midday is estimated to be 1 s. These findings provide a UV spectral signature of the strong ^ - transition associated with the four electrons on the carbonyl oxide group in CH₂OO. Extensions of this work to methyl substituted Criegee intermediates reveal similar absorption features arising from the COO -system .