2019MNRAS.490.2936M

The interaction of volatile species with carbonaceous interstellar dust analogues is of relevance in the chemistry and physics of dense clouds in the interstellar medium. Two deposits of hydrogenated amorphous carbon (HAC), with different morphologies and aromatic versus aliphatic ratio in their structure, have been grown to model interstellar dust. The interaction of N₂, CO, CH₄, and CO₂ with these two surfaces has been investigated using thermal programmed desorption (TPD). Desorption energy distributions were obtained by analysing TPD spectra for one monolayer coverage with the Polanyi-Wigner equation. The desorption energies found in this work for N₂, CO, and CH₄ are larger by 10-20 per cent than those reported in the literature for siliceous or amorphous solid water surfaces. Moreover, the experiments suggest that the interaction of the volatiles with the aromatic substructure of HAC is stronger than that with the aliphatic part. Desorption of CO₂ from the HAC surfaces follows zero-order kinetics, reflecting the predominance of CO₂-CO₂ interactions. A model simulation of the heating of cold cloud cores shows that the volatiles considered in this work would desorb sequentially from carbonaceous dust surfaces with desorption times ranging from hundreds to tens of thousands of years, depending on the molecule and on the mass of the core.