Abstract
Context. The UV photoreactivity of polycyclic aromatic hydrocarbons (PAHs) in porous amorphous solid water has long been known to form both oxygenated photoproducts and photofragments. Aims. The aim of this study is to examine the influence of ice structure on reactivity under soft UV irradiation conditions. Methods. Mixtures of PAHs with amorphous solid water (porous and compact) and crystalline (cubic and hexagonal) ices were prepared in a high vacuum chamber and irradiated using a mercury lamp for up to 2.5 h. Results. The results show that the production of oxygenated PAHs is efficient only in amorphous water ice, while fragmentation can occur in both amorphous and crystalline ices. We conclude that the reactivity is driven by PAH–water interactions in favourable geometries, notably where dangling bonds are available at the surface of pores. Conclusions. These results suggest that the formation of oxygenated PAH molecules is most likely to occur in interstellar environments with porous (or compact) amorphous solid water and that this reactivity could considerably influence the inventory of aromatics in meteorites.
Highlights
In interstellar objects, water ice is typically detected via its broad OH stretching absorption band at 3.1 μm (e.g. Smith et al 1989; Thi et al 2006)
It has been shown that the profile of the 3.1 μm band can be used to distinguish the four ice structures accessible under the diverse physical conditions of the interstellar medium and planetary systems, namely porous amorphous solid water, compact amorphous solid water, cubic crystalline ice (Ic), and hexagonal crystalline ice (Ih), as its profile is sensitive to the structure of the ice (e.g. Hagen et al 1981; Mastrapa et al 2009)
Water accreting from the gas phase onto cold (10–20 K) dust-grain surfaces in molecular clouds forms porous amorphous solid water (pASW), but water molecules are initially more likely to form in situ on grain surfaces via radical reactions (Tielens & Hagen 1982; Dulieu et al 2010) whose exothermicity may allow restructuring to compact amorphous solid water (cASW)
Summary
Water ice is typically detected via its broad OH stretching absorption band at 3.1 μm (e.g. Smith et al 1989; Thi et al 2006). Observations of crystalline ice beyond the snow line and below the photodesorption layer have been attributed to crystallisation induced by grain collisions (McClure et al 2015). In none of these environments is interstellar water ice present in its chemically pure form, because other small molecules are formed concurrently or may accrete onto dust grains, being incorporated into their icy mantles. Both the composition and structure is impacted by the inclusion of other molecular species
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