Reconstruction of water ice: the neglected process OH + OH → H₂O + O

¹ Sorbonne Université, UPMC Univ. Paris 06, UMR-CNRS 7616, Laboratoire de Chimie Théorique, 75252 Paris Cedex 05, France
e-mail: pauzat@lct.jussieu.fr
² Universidad de Valladolid, Facultad de Ciencias (Dpto. QF), Valladolid, Spain
e-mail: predondo@obs.uva.es

Received: 19 February 2020
Accepted: 4 May 2020

Abstract

Context. Although H₂O is the most important molecular material found in the solid state in the interstellar medium, the chemical routes leading to ice through surface reactions are still a matter of discussion. Three reaction pathways proposed in the past are at the heart of current research: hydrogenation of atomic oxygen, molecular oxygen, and ozone. The reaction network finally leads to a small number of processes giving H₂O: H + OH, H₂ + OH, and H + H₂O₂. To these processes, OH + OH should be added. It is known to be efficient in atmospheric chemistry and takes the irradiations of the interstellar grains into account that, directly or indirectly, create a number of OH radicals on and in the icy mantles.

Aims. We study the role of the existing ice in its own reconstruction after it is destroyed by the constant irradiation of interstellar grains and focus on the OH + OH reaction in the triplet state.

Methods. We used numerical simulations with a high level of coupled cluster ab initio calculations for small water aggregates and methods relevant to density functional theory for extended systems, including a periodic description in the case of solid water of infinite dimensions.

Results. OH + OH → H₂O + O reaction profiles are reported that take the involvement of an increasing number of H₂O support molecules into account. It is found that the top of the barrier opposing the reaction gradually decreases with the number of supporting H₂O and falls below the level of the reactants for H₂O layers or solid water.

Conclusions. In contrast to the gas phase, the reaction is barrierless on water ice. By adding a reconstructed H₂O molecule and a free oxygen atom at the surface of the remaining ice, this reaction leaves open the possibility of the ice reconstruction.