Knowledge Bank

The hydroperoxyl radical, $HO2$, is an important intermediate in many chemical reactions which occur in combustion, atmospheric chemistry and photochemical processes. In order to better understand the importance of this intermediate species, we have determined the UV photoabsorption spectrum, photodissociation products, and stratospheric photodissociation lifetimes of $HO2$ using theoretical methods. Adiabatic potential energy curves for the two lowest states of and symmetry as a function of the H-O-O bond angle, and the O-O and O-H bond lengths have been obtained from self-consistent-field plus configuration-interaction calculations. The resulting equilibrium geometries for the (1) and (1) electronic states are in good agreement with experiment. Both the (2) and (2) states are repulsive with respect to O-O stretch, and should dissociate rapidly to $H(2S)$ plus $O2(1\Delta g)$ and $O2(1\Sigma g+)2$, respectively. In contrast, potential maxima exist on both the (2) and (2) potential energy surfaces as the OH bond is lengthened. Therefore, the photodissociation products should be predominantly if not exclusively $H+O2$. Photodissociation cross sections as a function of wavelength were computed using theoretical electronic transition moments and the method of Gislason for treating bound-free transitions. The intense feature in the UV spectrum near 205 nm arises from the (1) - (2) transition and has a theoretical peak cross section which falls between the experimental results of Hochanadel, et al. and Paukhert and Johnston. The implications of these results on the lifetime and fate of stratospheric $HO2$ will be briefly discussed.