Energy Redistribution following CO$_2$ Formation on Cold Amorphous Solid Water

Abstract

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO$_2$ formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO($^1 \Sigma^+$) + O($^1$D) recombination the average translational and internal energy of the water molecules increases on the $\sim 10$ ps time scale by 15 % to 20 % depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO$_2$ and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid $\sim 10$ ps and a considerably slower $\sim 1$ ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the products generated.