A Theoretical Study of Temperature-dependent Photodissociation Cross Sections and Rates for O2

Abstract

The photodissociation of O _2 is thought to play a vital role in blocking UV radiation in the Earth’s atmosphere and likely has great importance in characterizing exoplanetary atmospheres. This work considers four photodissociation processes of O _2 associated with its four electronic states, whose potential energy curves and transition dipole moments are calculated at the icMRCI+Q/aug-cc-pwCV5Z-DK level of theory. The quantum-mechanical approach is used to compute the state-resolved cross sections for two triplet transitions from the ground X ${}^{3}{{\rm{\Sigma }}}_{{\rm{g}}}^{-}$ state to the excited B ${}^{3}{{\rm{\Sigma }}}_{{\rm{u}}}^{-}$ and E ${}^{3}{{\rm{\Sigma }}}_{{\rm{u}}}^{-}$ states, and for two singlet transitions from the a ^1 Δ _g and b ${}^{1}{{\rm{\Sigma }}}_{{\rm{g}}}^{+}$ states to the 1 ^1 Π _u state, with a consideration of photon wavelengths from 500 Å to the relevant threshold. Assuming the populations of the initial states satisfy a Boltzmann distribution, the temperature-dependent photodissociation cross sections are estimated at gas dynamic temperatures of 0–10,000 K, in which the discrete progressions of the B ${}^{3}{{\rm{\Sigma }}}_{{\rm{u}}}^{-}\leftarrow {\rm{X}}$ ${}^{3}{{\rm{\Sigma }}}_{{\rm{g}}}^{-}$ and E ${}^{3}{{\rm{\Sigma }}}_{{\rm{u}}}^{-}\leftarrow {\rm{X}}$ ${}^{3}{{\rm{\Sigma }}}_{{\rm{g}}}^{-}$ transitions are also considered. The photodissociation rates of O _2 in the interstellar, solar, and blackbody radiation fields are also calculated using the temperature-dependent cross sections. The resulting photodissociation cross sections and rates are important for the atmospheric chemistry of Earth and may be also useful for the atmospheric exploration of exoplanets.

Keywords

• Astrochemistry
• Molecule destruction
• Interstellar molecules
• Radiative processes