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 CO2 formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(1Σ+) + O(1D) recombination the average translational and internal energy of the water molecules increases on the ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO2 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 ps and a considerably slower 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 reaction products generated.
Highlights
The motion of adsorbates in and on amorphous solid water (ASW) is essential for chemistry at astrophysical conditions
All molecular dynamics (MD) simulations were carried out using the CHARMM suite of programs (Brooks et al, 2009) with provisions for bond forming reactions through multi state adiabatic reactive MD (MS-ARMD). (Nagy and Yosa Reyes, 2014)
Recombination on the surface leads to excess internal energy on the picosecond time scale which subsequently relaxes and additional energy transfer into water modes occurs on longer time scales
Summary
The motion of adsorbates in and on amorphous solid water (ASW) is essential for chemistry at astrophysical conditions. Bulk water is present in the form of ASW which is the main component of interstellar ices. The structure of ASW is usually probed by spectroscopic measurements (Hagen et al, 1981; Jenniskens and Blake, 1994) interferencebased methods have been employed. ASWs are porous structures characterized by surface roughness and internal cavities of different sizes which can retain molecular or atomic guests. Under laboratory conditions the water ices have been reported to be porous (He et al, 2016; Kouchi et al, 2020) or non-porous (Oba et al, 2009; He et al, 2016; Kouchi et al, 2020) ASW whereas the morphology of ices in the interstellar medium is more debated.
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