Overtone Dissociation of Peroxynitric Acid (HO 2 NO 2 ): Absorption Cross Sections and Photolysis Products †
Résumé
Band strengths for the second (3νOH) and third (4νOH) overtones of the OH stretch vibration of peroxynitric acid, HO2NO2 (PNA) in the gas-phase were measured using Cavity Ring-Down Spectroscopy (CRDS). Both OH overtone transitions show diffuse smoothly varying symmetrical absorption profiles without observable rotational structure. Integrated band strengths (base e) at 296 K were determined to be S3νOH ) (5.7 ( 1.1) × 10-20 and S4νOH ) (4.9 ( 0.9) × 10-21 cm2 molecule-1 cm-1 with peak cross sections of (8.8 ( 1.7) × 10-22 and (7.0 ( 1.3) × 10-23 cm2 molecule-1 at 10086.0 ( 0.2 cm-1 and 13095.8 ( 0.4 cm-1, respectively, using PNA concentrations measured on line by Fourier-transform infrared and ultraviolet absorption spectroscopy. The quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors in the measurements. OH overtone spectra measured at lower temperature, 231 K, showed a narrowing of the 3νOH band along with an increase in its peak absorption cross section, but no change in S3νOH to within the precision of the measurement ((9%). Measurement of a PNA action spectrum showed that HO2 is produced from second overtone photodissociation. The action spectrum agreed with the CRDS absorption spectra. The PNA cross sections determined in this work for 3νOH and 4νOH will increase calculated atmospheric photolysis rates of PNA slightly.