  The UV photoreactivity of polycyclic aromatic hydrocarbons (PAHs) in porous
amorphous solid water has long been known to form both oxygenated photoproducts
and photofragments. The aim of this study was to examine the influence of the
ice structure upon reactivity under soft UV irradiation conditions. 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 hours. 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. 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.
