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

Abstract

The photodissociation of O2 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 O2 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 Σ g − state to the excited B 3 Σ u − and E 3 Σ u − states, and for two singlet transitions from the a 1Δg and b 1 Σ g + states to the 1 1Πu 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 Σ u − ← X 3 Σ g − and E 3 Σ u − ← X 3 Σ g − transitions are also considered. The photodissociation rates of O2 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.