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 for 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 that the molecular species stabilize through vibrational relaxation. The relative populations of the species is 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$_2$ is expected to modify the chemical evolution of cold interstellar ($T \sim 10 - 50$ K) and warmer noctilucent ($T \sim 100$ K) clouds.