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

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 (4ν OH ) and fourth (5ν OH ) overtone transitions in these molecules. The integrated cross sections are (2.25 ± 0.15) × 10 -21 and (2.57 ± 0.24) × 10 -22 cm 2 molecule -1 cm -1 for 4ν OH and 5ν OH in nitric acid, respectively, and (4.58 ± 0.39) × 10 -21 and (5.67 ± 0.52) × 10 -22 cm 2 molecule -1 cm -1 for 4ν OH and 5ν OH in hydrogen peroxide. For both molecules, our report is the first direct intensity measurement for 5ν OH, 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 HNO 3 and H 2 O 2 is small but not completely negligible.