An embedded cluster study of the formation of water on interstellar dust grains

The formation of water in the interstellar medium from hydrogen and oxygen atoms on a pristine olivine surface (forsterite (010)) is investigated with an embedded cluster approach. The 55-atom quantum cluster is described at the density functional level while the remaining 1629 atoms of the surface cluster are described with atomistic potentials. Transition states are most easily calculated with our modified implementation of the climbing-image nudged elastic band method in ChemShell. With these computational techniques, we find that gas phase hydrogen atoms can chemisorb (−102 kJ mol−1) without an activation barrier on the forsterite (010) surface, concomitantly creating a surface electron at the adjacent magnesium atom site. Subsequently, an oxygen atom chemisorbs strongly to this surface electron site (−432 kJ mol−1). The rearrangement of the adjacently chemisorbed O and H to a chemisorbed OH-radical is endothermic by 4 kJ mol−1 and activated by 27 kJ mol−1. This chemisorbed OH can then react barrierlessly with a second hydrogen atom to yield adsorbed water (−511 kJ mol−1). Alternatively, if O and H do not recombine to form OH, but instead thermally equilibrate, a second hydrogen atom can react with the chemisorbed oxygen atom (−501 kJ mol−1) to yield dissociatively adsorbed water (OH− and H+), which then can rearrange to associatively adsorbed water (−5 kJ mol−1, ΔE‡ = 18 kJ mol−1) or gas phase water (+91 kJ mol−1). The formation of water on a bare dust grain from hydrogen and oxygen atoms is thus catalysed by an olivine surface by stabilising the reaction intermediates and product. Since the reaction proceeds via three chemisorbed intermediates, thermal equilibration is facilitated and back-dissociation of the freshly formed reaction products OH and H2O would not occur as frequently as it would in the gas phase or when the reactants are physisorbed on a surface.