Two-photon excitation of hydrogen peroxide at 266 and 193 nm: Determination of the absorption cross section and photofragment state distribution

Abstract

The sequential two-photon dissociation of hydrogen peroxide into electronically excited hydroxyl radicals has been investigated at the photolysis wavelengths of 266 and 193 nm. Using a conventional rate equation approach, the differential second-photon absorption cross section at 266 nm was determined to be σ2 ≈ 2 × 10-16 cm2 by a comparison of the photoinduced OH (A 2Σ+) emission with the laser-induced fluorescence of simultaneously formed OH (X 2Πi) under the same excitation and detection conditions. The second-photon absorption cross section at 193 nm was found to be smaller by a factor of approximately 60, in accordance with the existence of additional dark dissociation channels. The analysis of the distribution of available energy into OH (A) reveals that translational excitation must be very high and, furthermore, that there is substantially less energy left in fragment rotation than expected from results of isoenergetic one-photon dissociation. The rotational state distributions in ν′ = 0 are found to be approximately Boltzmannian with maximum populations at N′OH ≈ 6 (266 nm) and N′OH ≈ 9 (193 nm). The internal energy distribution of OH (A) resembles that of OH (X) following the one-photon process at the same wavelength and half of the photon energy. Thus, a model is proposed, in which the two-photon dissociation dynamics is characterized by the evolution of the intermediate state departing from the first-photon Franck-Condon region before further absorption. © 1989.