An infrared measurement of chemical desorption from interstellar ice analogues

Abstract

In molecular clouds at temperatures as low as 10 K, all species except hydrogen and helium should be locked in the heterogeneous ice on dust grain surfaces. Nevertheless, astronomical observations have detected over 150 different species in the gas phase in these clouds. The mechanism by which molecules are released from the dust surface below thermal desorption temperatures to be detectable in the gas phase is crucial for understanding the chemical evolution in such cold clouds. Chemical desorption, caused by the excess energy of an exothermic reaction, was first proposed as a key molecular release mechanism almost 50 years ago. Chemical desorption can, in principle, take place at any temperature, even below the thermal desorption temperature. Therefore, astrochemical net- work models commonly include this process. Although there have been a few previous experimental efforts, no infrared measurement of the surface (which has a strong advantage to quantify chemical desorption) has been performed. Here, we report the first infrared in situ measurement of chemical desorption during the reactions H + H2S -> HS + H2 (reaction 1) and HS + H -> H2S (reaction 2), which are key to interstellar sulphur chemistry. The present study clearly demonstrates that chemical desorption is a more efficient process for releasing H2S into the gas phase than was previously believed. The obtained effective cross-section for chemical desorption indicates that the chemical desorption rate exceeds the photodesorption rate in typical interstellar environments.

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• Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets
• Gas-phase formation of acetaldehyde: review and new theoretical computations
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• Successive H-atom addition to solid OCS on compact amorphous solid water
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• Processing of hydroxylamine, NH2OH, an important prebiotic precursor, on interstellar ices
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• Formation of carbonyl sulfide (OCS) via SH radicals in interstellar CO-rich ice under dense cloud conditions
• Photochemistry of interstellar ice forming Complex Organic Molecules
• A multigrain-multilayer astrochemical model with variable desorption energy for surface species
• H2S ice sublimation dynamics: experimentally constrained binding energies, entrapment efficiencies, and snowlines
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• Neural Network Potential Energy Surface for the low temperature Ring Polymer Molecular Dynamics of the H2CO + OH reaction