Infrared action spectroscopy and dissociation dynamics of the HOOO radical

The HOOO radical has long been postulated to be an important intermediate in atmospherically relevant reactions and was recently deemed a significant sink for OH radicals in the tropopause region. In the present experiments, HOOO radicals are generated in a pulsed supersonic expansion by the association of O₂ and photolytically generated OH radicals, and the spectral signature and vibrational predissociation dynamics are investigated via IR action spectroscopy, an IR-UV double resonance technique. Rotationally resolved IR action spectra are obtained for trans-HOOO in the fundamental (ν_OH) and overtone (2ν_OH) OH stretching regions at 3569.30 and 6974.18 cm^-1, respectively. The IR spectra exhibit homogeneous line broadening, characteristic of a ∼26-ps lifetime, which is attributed to intramolecular vibrational redistribution and/or predissociation to OH and O₂ products. In addition, an unstructured feature is observed in both the OH fundamental and overtone regions of HOOO, which is likely due to cis-HOOO. The nascent OH X²Π, v = 0 or v = 1, products following vibrational predissociation of HOOO, ν_OH or 2ν_OH, respectively, have been investigated using saturated laser-induced fluorescence measurements. A distinct preference for population of Π(A‘) Λ-doublets in OH was observed and is indicative of a planar dissociation of trans-HOOO in which the symmetry of the bonding orbital is maintained