Formation and Stabilization of Ground and Excited-State Singlet O-2 upon Recombination of P-3 Oxygen on Amorphous Solid Water

The recombination dynamics of P-3 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 Sigma(-)(g)) O-2 and molecular oxygen in the two lowest singlet states (a(1)Delta(g) and b(1)Sigma(+)(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 similar to 10-50 K) and warmer noctilucent (T similar to 100 K) clouds.