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

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.

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Physisorption and desorption of H2, HD and D2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites

Physical Chemistry Chemical Physics ◽

10.1039/c5cp03985a ◽

2015 ◽

Vol 17 (44) ◽

pp. 30148-30157 ◽

Cited By ~ 8

Author(s):

Lionel Amiaud ◽

Jean-Hugues Fillion ◽

François Dulieu ◽

Anouchah Momeni ◽

Jean-Louis Lemaire

Keyword(s):

Molecular Hydrogen ◽

Amorphous Solid ◽

Statistical Population ◽

Adsorption And Desorption ◽

Amorphous Solid Water ◽

Water Ice ◽

Adsorption Sites ◽

Solid Water

We study the adsorption and desorption of three isotopologues of molecular hydrogen mixed on 10 ML of porous amorphous water ice (ASW) deposited at 10 K.

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Diffusion of CH4 in amorphous solid water

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038705 ◽

2020 ◽

Vol 643 ◽

pp. A163

Author(s):

Belén Maté ◽

Stéphanie Cazaux ◽

Miguel Ángel Satorre ◽

Germán Molpeceres ◽

Juan Ortigoso ◽

...

Keyword(s):

Monte Carlo ◽

Monte Carlo Simulations ◽

Diffusion Coefficients ◽

Amorphous Solid ◽

Amorphous Solid Water ◽

Water Ice ◽

Cometary Material ◽

Order Of Magnitude ◽

Diffusion Rates ◽

Solid Water

Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H2O and CH4 were grown in different conditions and the sublimation of CH4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick’s law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10−12 and 10−13 cm2 s−1 for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick’s laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered.

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