Formation and Stabilization of Ground and Excited-State Singlet O2 upon Recombination of 3P Oxygen on Amorphous Solid Water

The recombination dynamics of 3 P oxygen atoms on cold amorphous solid water to form triplet and singlet molecular oxygen (O 2 ) is investigated under conditions representative of cold clouds. Reactive molecular dynamics simulations including Landau–Zener-based hopping to account for nonadiabatic transitions find that both ground-state ( X 3 Σ g – ) O 2 and molecular oxygen in the two lowest singlet states ( a 1 Δ g and b 1 Σ g + ) can be formed and the molecular species stabilize through vibrational relaxation. The relative populations of the species are approximately 1:1:1. These results also agree qualitatively with a kinetic model based on simplified wavepacket simulations. The presence and stabilization of higher electronic states of O 2 are expected to modify the chemical evolution of cold interstellar ( T ∼ 10–50 K) and warmer noctilucent ( T ∼ 100 K) clouds.