Energy Redistribution Following CO2 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 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 ∼10 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 ∼10 ps and a considerably slower ∼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 reaction products generated.

Quantitative molecular simulations

“…For the process in gas phase all necessary interactions are accounted for in one global energy function 190 whereas for the process on amorphous solid water the reorganization of the electrons between the CO + O reactant and the CO 2 product needs to be included explicitly. 45,269…”

“…For the process in gas phase all necessary interactions are accounted for in one global energy function 190 whereas for the process on amorphous solid water the reorganization of the electrons between the CO + O reactant and the CO 2 product needs to be included explicitly. 45,269 Conformationally dependent electrostatics has already been explored to some extent in the past. 270 However, the applications were often restricted to water 93 with the exception of the fluctuating charge implementation in CHARMM.…”

“…Recombination simulations were initiated from a CO(center of mass)-O separation of (i) R = 4.0 Å for a previously used 45 fixed point charge model in the collinear OC-O (y = 1801) conformation and (ii) for separations ranging from R = 4.0 Å to R = 8.0 Å with a fluctuating charge model. With fixed charges 45,269 the oxygen atom interacts more strongly with the water hydrogen atoms due to its charge of q O = À0.2 e. Although some charge transfer between the water surface and the oxygen atom is expected from electronic structure calculations 45 the magnitude of the charge is probably too high. On the other hand, with the fluctuating charge model it is possible to correctly describe the change in the charges of the CO and O moieties as they approach one another.…”

Quantum and quasi-classical dynamics of the C(3P) + O2(3Σ−g) → CO(1Σ+) + O(1D) reaction on its electronic ground state

“…The thermoneutral atom-exchange reaction (CO A + O B → CO B + O A ) has already been considered in the context of stabilizing CO 2 on amorphous solid water (ASW) to better understand molecule formation at astrophysical conditions. 70,71 For this process it was found that for bulk temperatures of ∼ 50 K formation and stabilization of CO 2 is very likely (60 % of the trajectories stabilize). Together with the present results this suggests that quantum effects will only marginally affect this conclusion.…”

Quantitative Molecular Simulations

“…For the process in gas phase all necessary interactions are accounted for in one global energy function 163 whereas for the process on amorphous solid water the reorganization of the electrons between the CO+O reactant and the CO 2 product needs to be included explicitly. 37,243 Conformationally dependent electrostatics has already been explored to some extent in the past. However, the applications were often restricted to water 85 with the exception of the fluctuating charge implementation in CHARMM.…”