Formation and Stabilization of Ground and Excited State Singlet O${}_2$ upon Recombination of ${}^3$P Oxygen on Amorphous Solid Water

Abstract

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 ($X^3{\Sigma }_g^{-}$) O${}_2$ and molecular oxygen in the two lowest singlet states ($a^1{\Delta }_g$ and $b^1{\Sigma }_g^{+}$) 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 $a^1{\Delta }_g$ and $b^1{\Sigma }_g^{+}$ oxygen influences the chemical evolution of cold clouds.