15min:

ILANA B. POLLACK, EUNICE X. J. LI, IAN M. KONEN AND MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

The OH + NO₂ + M HONO₂ + M reaction is of fundamental importance in atmospheric chemistry because it is an important sink of reactive HO_x and NO_x radicals that directly affect the destruction of ozone in the stratosphere and the production of ozone in the troposphere. Peroxynitrous acid (HOONO), a secondary product of the OH + NO₂ reaction and a less stable isomer of HONO₂, has been created in a pulsed supersonic free-jet expansion by reaction of photolytically generated OH radicals with NO₂. An infrared pump-ultraviolet probe technique has been used to spectroscopically characterize the trans-perp (tp) conformer of HOONO. Infrared action spectra of tp-HOONO have been recorded in the OH overtone region near 1.4 µm using a single-mode optical parametric oscillator. Rotational structure of the pure OH overtone band has been observed at 6971.4 cm^-1 (band origin). Experimental spectra have been assigned by comparison to simulated rotational band structure using rotational constants and estimated vibrational frequencies predicted by previous ab initio calculations. Infrared excitation of tp-HOONO in the OH overtone region provides sufficient energy to break the O--O bond leading to unimolecular dissociation into OH and NO₂ products in less than 6 ns (laser-limited). An OH (v=0) product state distribution was obtained following infrared overtone excitation. The highest populated OH ² _3/2 (v=0) state observed, J=15/2, establishes a tp-HOONO binding energy of D₀ \leq 17 kcal/mol, which is consistent with current theoretical estimates of the binding energy.