Formation and Stabilization of Ground and Excited State Singlet O$_2$ upon Recombination of $^3$P 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 followed under conditions representative for cold clouds. It is found that both, formation of ground state ($X ^3 Σ_{g}^{-}$) O$_2$ and molecular oxygen in the two lowest singlet states ($a ^1Δ_g$ and $b ^1Σ_g^+$) is possible and that the species can stabilize. The relative proportions of the species is approximately 1:1:1. These results also agree qualitatively with a kinetic model based on simplified wavepacket simulations. As the chemical reactivity of triplet and singlet O$_2$ is different it is likely that substantial amounts of $a ^1Δ_g$ and $b ^1Σ_g^+$ oxygen influences the chemical evolution of cold clouds.

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