An Infrared Measurement of Chemical Desorption From Interstellar Ice Analogues

FTIR spectra of samples. a. Solid H2S (=0.7 monolayer) deposited on ASW (~30 monolayers). The inset shows an enlarged spectrom focusing on the S-H stretching band of H2S (2,570 cm-1) 18. Difference spectra after exposure to H atoms for 20 (top), 60 (middle) and 120 min (bottom). b. The spectral change at around 2,300 cm-1 is due to the temperal variation of atmospheric CO2 concentration in the infrared beam line outside the vacuum chamber.

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 network 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.

Y. Oba, T. Tomaru, T. Lamberts, A. Kouchi, N. Watanabe
(Submitted on 10 Oct 2018)

Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Journal reference: Oba et al. (2018) Nature Astronomy, 2, 228-232
DOI: 10.1038/s41550-018-0380-9
Cite as: arXiv:1810.04669 [astro-ph.GA] (or arXiv:1810.04669v1 [astro-ph.GA] for this version)
Submission history
From: Yasuhiro Oba
[v1] Wed, 10 Oct 2018 05:24:11 GMT (714kb)

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