Theoretical study of the four lowest doublet electronic states of the hydroperoxyl radical: Application to photodissociation

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Abstract
Potential‐energy curves have been calculated for the four lowest doublet states of the hydroperoxyl radical as a function of the H–O–O bond angle and the oxygen–oxygen and oxygen–hydrogen bond lengths. A polarized double‐zeta basis of Cartesian–Gaussian functions was used. An extensive configuration–interaction treatment and subsequent extrapolation procedure was used to obtain a uniform description of the four electronic states over a wide range of nuclear geometries. The calculated equilibrium oxygen–oxygen bond lenths for the ground state, (1) 2A″, and the low‐lying (1) 2A′ excited state are in excellent agreement with the values determined from absorption spectra in the near infrared. An analysis of the shapes of the potential curves for the (2) 2A″ and (2) 2A′ states, as a function of the oxygen–oxygen and oxygen–hydrogen bond lengths, indicates that OH and atomic oxygen should be the predominant photodissociation products. Calculated dipole moments are reported for each state. These results indicate that the (2) 2A″ and (2) 2A′ states are quite ionic. The (1) 2A′← (1) 2A″ transition in the near infrared is calculated to be quite weak, giving a radiative lifetime for the (1) 2A″ state of about 1 msec. The most intense transition is (2) 2A″← (1) 2A″ and it accounts for the broad continuum feature observed in the UV absorption spectrum with a maximum near 210 nm. Absorption cross sections for HO2 as a function of wavelength are calculated form the theoretical data and are found to be in excellent agreement with the measurements of Paukert and Johnston. The theoretical results support the HO2 photodissociation data that are at present used at Ames Research Center in model studies of ozone depletion.