Absolute intensities for third and fourth overtone absorptions in HNO3 andH2O2 measured by cavity ring down spectroscopy

Photodissociation of nitric acid and hydrogen peroxide via high-lying O-H o vertone absorptions in the visible may act as a source of OH radicals in th e atmosphere. We have used cavity ring down spectroscopy to measure the abs orption cross sections for the third (4 nu(OH)) and fourth (5 nu(OH)) overt one transitions in these molecules. The integrated cross sections are (2.25 +/- 0.15) x 10(-21) and (2.57 +/- 0.24) x 10(-22) cm(2) molecule(-1) cm(-1 ) for 4 nu(OH) and 5 nu(OH) in nitric acid, respectively, and (4.58 +/- 0.3 9) x 10(-21) and (5.67 +/- 0.52) x 10(-22) cm(2) molecule(-1) cm(-1) for 4 nu(OH) and 5 nu(OH) in hydrogen peroxide. For both molecules, our report is the first direct intensity measurement for 5 nu(OH). the lowest dissociati ve overtone transition. We compare our values for the lower overtones to th ose from previous studies, where available. Our measured cross sections sug gest that the contribution of direct overtone excitation to the atmospheric photodissociation of HNO3 and H2O2 is small but not completely negligible.