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

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 hours. 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.

Enhanced reactivity of oxygen-functionalised PAHs with atomic hydrogen – A route to the formation of small oxygen-carrying molecules

“…In cold regions of the ISM, PAHs are expected to be frozen out on dust grains. Laboratory experiments investigating the processing of water-ice and PAH mixtures by energetic radiation such as UV light, electrons, or protons show that superhydrogenated PAHs are produced, alongside PAHs with alcohol and ketone functional groups indicating that such oxygen-bearing PAH species could be formed in the ISM on icy grains (Bernstein et al 1999;Gudipati & Yang 2012;Bouwman et al 2011;Guennoun et al 2011a,b;Noble et al 2020). Cook et al (2015) estimate that interstellar ices could contain neutral and ionised PAHs, alcohols, ketones, and quinones at the 2-4% level relative to water.…”

Visible-light photoionization of aromatic molecules in water-ice: Organic chemistry across the universe with less energy

“…An important opening of such direction is a recent work by Noble et al on PAH photochemistry in different ice structures. [22] Excluding Multi-photon Ionization Due to a possibility of multi-photon ionization under pulsed laser irradiation, it was important to show that our results were due to an absorption of a single photon since multiphoton ionization is significantly less likely to happen in nature. We did confirm this with three independent ways shown below.…”

Infrared multiple‐angle incidence resolution spectrometry for vapor‐deposited amorphous water

“…Dangling OH bonds have been intensely studied to understand the surface properties of interstellar ice and its catalytic role in the formation of molecules in space, because they reflect the disorderliness of the ice surface structure as well as the ice's porosity, and also serve as catalytic sites for adsorption and chemical reactions. [5,70,84,86,87] Despite much active research, the absorption cross-section, a fundamental optical constant, remains unknown for dangling OH bonds. This hampers the quantification of dangling OH bonds from the IR spectra of ice, ultimately preventing a molecular-level understanding of the ice surface.…”