Influence of the ice structure on the soft UV photochemistry of PAHs embedded in solid water

TL;DR

This study investigates how different ice structures affect the UV-induced chemical reactions of PAHs in solid water, revealing that porous amorphous ice promotes oxygenated PAH formation, impacting astrochemical processes.

Contribution

It provides new insights into how ice structure influences PAH photochemistry, highlighting the role of pore surface interactions in interstellar environments.

Findings

Oxygenated PAHs form efficiently only in amorphous ice.

Fragmentation occurs in both amorphous and crystalline ices.

Porous ice promotes PAH-water interactions at pore surfaces.

Abstract

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.