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

P Redondo,Y Ellinger,F Pauzat,A Markovits

doi:10.1051/0004-6361/202037771

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.

Highlights

Water ice has been found throughout the cold interstellar medium (ISM; Merrill et al 1976; Whittet et al 1988; Dartois et al 1998; Gibb et al 2004; Boogert et al 2008), the chemical origin and longevity of the icy grain mantles in which H2O is the dominant component is still debated
The three reaction pathways proposed by Tielens & Hagen (1982), that is, hydrogenation of atomic oxygen, molecular oxygen, and ozone, are at the origin of the chemical models and inspired most of the laboratory experiments
The hydrogen atoms produced by the diffusing irradiation give mostly molecular H2. These hydrogen molecules could react with the OH radicals (reaction (2)), and it has been shown that this reaction could proceed through the tunneling effect (Oba et al 2012; Meisner et al 2017)

Summary

Introduction

Water ice has been found throughout the cold interstellar medium (ISM; Merrill et al 1976; Whittet et al 1988; Dartois et al 1998; Gibb et al 2004; Boogert et al 2008), the chemical origin and longevity of the icy grain mantles in which H2O is the dominant component is still debated. Because the molecule formation in the gas phase is not very efficient, on-grain processes have to play a major role in the survival of the ice mantles. These on-grain mechanisms have been proposed in chemical models in the 1980s and are still discussed today (Tielens & Hagen 1982; d’Hendecourt et al 1985; Hasegawa & Herbst 1993; Cuppen & Herbst 2007). The three reaction pathways proposed by Tielens & Hagen (1982), that is, hydrogenation of atomic oxygen, molecular oxygen, and ozone, are at the origin of the chemical models and inspired most of the laboratory experiments. The last steps of the reaction network, which are those that effectively yield H2O, are

Objectives

Conclusion