Absolute Intensities for Third and Fourth Overtone Absorptions in HNO₃ and H₂O₂ Measured by Cavity Ring Down Spectroscopy

Abstract

Photodissociation of nitric acid and hydrogen peroxide via high-lying O−H overtone absorptions in the visible may act as a source of OH radicals in the atmosphere. We have used cavity ring down spectroscopy to measure the absorption cross sections for the third (4νOH) and fourth (5νOH) overtone transitions in these molecules. The integrated cross sections are (2.25 ± 0.15) × 10-21 and (2.57 ± 0.24) × 10-22 cm2 molecule-1 cm-1 for 4νOH and 5νOH in nitric acid, respectively, and (4.58 ± 0.39) × 10-21 and (5.67 ± 0.52) × 10-22 cm2 molecule-1 cm-1 for 4νOH and 5νOH in hydrogen peroxide. For both molecules, our report is the first direct intensity measurement for 5νOH, the lowest dissociative overtone transition. We compare our values for the lower overtones to those from previous studies, where available. Our measured cross sections suggest that the contribution of direct overtone excitation to the atmospheric photodissociation of HNO3 and H2O2 is small but not completely negligible.