TIMOTHY D. SECHLER, CRAIG MURRAY, ERIKA L. DERRO AND MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

The existence of hydrogen trioxy radicals (HO3) has several important implications in atmospheric chemistry, as it may act as an intermediate in key chemical reactions and relaxation processes in the atmosphere. In this work, HO3 was produced from the association of O_2 and photolytically generated OH radicals in the collisional region of a supersonic free-jet expansion. Excitation of the OH stretching vibration of HO3 via tunable infrared radiation causes dissociation to OH + O2, and the OH fragments are probed by laser-induced fluorescence. The IR action spectrum of HO3 was recorded for the fundamental and first overtone OH stretches of HO3 as well as its deuterated analog. Rotationally structured and broadened features were observed and attributed to the trans and cis conformers of HO3. The product state distribution of the OH fragment was examined to determine the highest energetically open product channel, which sets an upper limit of 6.12 kcal mol-1 for the HO-O2 binding energy. Calculation of the equilibrium constant for the reaction OH + O2 + M \rightleftharpoons HO3 + M (with collision partner M) suggests that HO3 exists in significant abundance in the vicinity of the tropopause, where it may act as a temporary sink for OH.