NO2 Quantum Yield from the 248 nm Photodissociation of Peroxynitric Acid (HO2NO2)
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Abstract:
Peroxynitric acid (PNA) was photolyzed at 248 nm, and the NO2 photoproduct was detected by laser-induced fluorescence (LIF). The quantum yield for the production of NO2 was determined by comparison with HNO3 photolysis data taken under identical experimental conditions. Measurements made over a range of pressures, flows, and precursor concentrations resulted in an NO2 quantum yield of 0.56 ± 0.17, where the statistical uncertainty is 2 standard deviations. Calculations of potential energy curves for several low-lying singlet and triplet states of PNA are presented. The calculations show that while the singlet excitations occur via an n−π* transition on the NO2 moiety, the dissociative channels forming OH + NO3 and HO2 + NO2 likely occur via predissocation on different surfaces. Excitation energies at the MRCI and CCSD(T) level of theory show that excited states of PNA are not accessible at wavelengths longer than 407 nm (∼3.0 eV).Keywords:
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The photodissociation spectrum of Ar+3 between 520 and 620 nm is reported. A broadband peaking near 520 nm is observed with a cross section of ∼1.8×10−16 cm2 at the peak, in agreement with the findings of Levinger et al. [J. Chem. Phys. 89, 71 (1988)]. However, in the present work, a shoulder whose prominence is highly temperature dependent is observed between 545 and 555 nm. The new results are discussed in terms of various electronic transitions and pathways to dissociation. The photodissociation mechanisms are studied by translational energy analysis of photofragmentation. Three photodissociation mechanisms that involve two electronic energy surfaces are proposed which account for the experimental findings.
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The quantum yield for the production of NO 3 in the N 2 O 5 photolysis at 248 nm was measured to be unity. The concentration of NO 3 produced upon photolysis was measured using 662‐nm laser absorption. The quantum yields for O( 3 P ) production in the photolysis of N 2 O 5 at 248 nm, 266 nm, 287 nm, and 289 nm were also measured using resonance fluorescence detection of O( 3 P ). The quantum yield for O( 3 P ) decreased from 0.7 at 248 nm to 0.15 at 289 nm. It is postulated that N 2 O 5 photolysis at wavelengths less than 298 nm yields NO 3 + NO + O( 3 P )in addition to NO 3 + NO 2 .
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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.
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