Diffusion of CH₄in amorphous solid water

Abstract

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 CH 4 on amorphous solid water (ASW) and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H 2 O and CH 4 were grown in different conditions and the sublimation of CH 4 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 cm 2 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 law 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.

Neural-Network Assisted Study of Nitrogen Atom Dynamics on Amorphous Solid Water -- II. Diffusion

“…Running the kMC simulations at T = 50 K, we obtain a diffusion coefficient of a nitrogen atom on ASW of 9.2×10 −9 cm 2 s −1 , which is already low, but still even higher than the experimental results (Maté, Belén et al 2020) obtained for CH 4 , a molecule with very similar weight and affinity with the ASW to N, of 10 −12 cm 2 s −1 . The influence of the temperature is severe, at T = 17 K we obtain D = (6.9±2.2)×10 −21 cm 2 s −1 , which is in much better agreement with estimates by He et al (He et al 2018) of 10 −20 cm 2 s −1 for CH 4 on ASW.…”

On the spectral features of dangling bonds in CH4/H2O amorphous ice mixtures

“…The experimental setup has been described in previous studies. 12 It consists of a high vacuum chamber, with a base pressure in the 10 À8 mbar range, that is provided with a closed cycle He cryostat and coupled to an FTIR spectrometer through KBr windows. A Si plate attached to the cold head of the cryostat was the surface where the ices were grown.…”

“…11 Before sublimation from CH 4 :H 2 O ice mixtures, CH 4 molecules diffuse through the pores of ASW interacting with the dangling bonds. 12 During this trip they can react with other trapped species or with the ice surface, making it possible to form species such as methanol. Understanding the mechanisms of methane-water interaction is a key to comprehend volatile retention in porous structures and to explore possible chemical reaction paths.…”

“…The present work focuses on the DB features of CH 4 /H 2 O mixtures, where, besides the above-mentioned pure ASW DB bands, a new peak appears, 13,14 shifted to lower wavenumber and with stronger intensity (henceforth termed DBM). This paper presents an ab initio study on spectral characteristics of DB and DBM modes.…”

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

“…Usually, astrochemical models assume that the radical E di f f is a fraction f of E des and the value f is derived from computations and experiments on species such as CO, CO 2 , H 2 O, CH 4 and NH 3 (e.g., Mispelaer et al 2013;Karssemeijer & Cuppen 2014;Lauck et al 2015;Ghesquière et al 2015;He et al 2017;Cooke et al 2018;He et al 2018;Maté et al 2020;Kouchi et al 2020). These studies give a value for f between 0.3 and 0.6, as mentioned in Sect.…”