Photodissociation of H2O2 and CH3OOH at 248 nm and 298 K: Quantum yields for OH, O(3P) and H(2S)

The quantum yields of the products, OH(X 2Π), O(3P) [plus O(1D)] and H(2S), in the photolysis of H2O2 and CH3OOH at 248 nm and 298 K have been measured. OH was directly observed by laser-induced fluorescence while the atomic species were detected by cw-resonance fluorescence. All quantum yield measurements were made using relative methods. The quantum yields of OH, O, and H in H2O2 photolysis were measured relative to the well known quantum yields of O(1D) and O(3P) in the photodissociation of O3, and H(2S) in CH3SH. The values we obtain are, 2.09±0.36, <0.002 and <0.0002 for OH, O, and H, respectively. For CH3OOH photolysis, the quantum yield of OH was measured relative to our value for OH quantum yield in H2O2 photolysis, and the quantum yields of O and H relative to those in O3 and CH3SH photodissociation, respectively. The values we obtain are, 1.00±0.18, <0.007 and 0.038±0.007 for OH, O, and H, respectively. In both H2O2 and CH3OOH photolysis, the observed O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH we measure is consistent with the known continuous and unstructured absorption spectra of these molecules in this wavelength region, where the most important process is the dissociative (Ã 1A←X̃ 1A) transition to give OH(X 2Π, v″=0) fragment.