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

¹ CNRS, Aix-Marseille Université, Laboratoire PIIM, Marseille, France
e-mail: jennifer.noble@univ-amu.fr
² Laboratoire de Chimie et Physique Quantiques LCPQ/IRSAMC, Université de Toulouse and CNRS, UT3-Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
³ Univ. Bordeaux, CNRS, Bordeaux INP, Institut des Sciences Moléculaires UMR 5255, 33405 Talence, France

Received: 3 June 2020
Accepted: 8 October 2020

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.