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 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 CH2OO. Extensions of this work to methyl substituted Criegee intermediates reveal similar absorption features arising from the COO pi -system .