Abstract
Experiments designed to reveal the low-temperature reactivity on the surfaces of interstellar dust grains are used to probe the heterogeneous reaction between oxygen atoms and acrylonitrile (C2H3CN, H2C=CH-CN). The reaction is studied at a series of fixed surface temperatures between 14 and 100 K. After dosing the reactants on to the surface, temperature-programmed desorption, coupled with time-of-flight mass spectrometry, reveals the formation of a product with the molecular formula C3H3NO. This product results from the addition of a single oxygen atom to the acrylonitrile reactant. The oxygen atom attack appears to occur exclusively at the C=C double bond, rather than involving the cyano(-CN) group. The absence of reactivity at the cyano site hints that full saturation of organic molecules on dust grains may not always occur in the interstellar medium. Modelling the experimental data provides a reaction probability of 0.007 ± 0.003 for a Langmuir–Hinshelwood style (diffusive) reaction mechanism. Desorption energies for acrylonitrile, oxygen atoms, and molecular oxygen, from the multilayer mixed ice their deposition forms, are also extracted from the kinetic model and are 22.7 ± 1.0 kJ mol−1 (2730 ± 120 K), 14.2 ± 1.0 kJ mol−1 (1710 ± 120 K), and 8.5 ± 0.8 kJ mol−1 (1020 ± 100 K), respectively. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and acrylonitrile could occur on interstellar dust grains on an astrophysical time-scale.
Highlights
The known universe is composed overwhelmingly of hydrogen and helium
Whenever acrylonitrile and O atoms are co-dosed on to the graphite surface, at surface temperatures below 100 K, a signal at m/z = 69 is observed in the mass spectra recorded during the temperature-programmed desorption (TPD) phase (Fig. 2)
The signal at m/z = 69 is consistent with a species with the empirical formula C3H3NO, which indicates the addition of a single oxygen atom to an acrylonitrile molecule
Summary
The known universe is composed overwhelmingly of hydrogen and helium. Yet, despite this composition, around 200 different molecular species have been detected to date in the interstellar medium (ISM). The destructive photons are completely absorbed by the molecules and dust making up the outer layers of these clouds, and the starlight cannot reach molecules located towards the clouds’ centres This absence of UV light, and the associated low rates of molecular photodissociation, means the gas temperature in dense interstellar clouds is comparable to the dust temperature (approximately 10–20 K) and molecular lifetimes are considerably extended in comparison with many other regions of the ISM. These extended molecular lifetimes, coupled with the low temperatures, allow icy mantles to form on the surface of the silicaceous and carbonaceous dust grains in the cloud; such grains typically make up about 1 per cent of a cloud’s mass (Williams & Cecchi-Pestellini 2016). These icy mantles are primarily composed of water but can include CO, CO2, and a variety of simple organic molecules (e.g. methanol)
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