FORMATION OF S-BEARING SPECIES BY VUV/EUV IRRADIATION OF H₂S-CONTAINING ICE MIXTURES: PHOTON ENERGY AND CARBON SOURCE EFFECTS

Abstract

Carbonyl sulfide (OCS) is a key molecule in astrobiology that acts as a catalyst in peptide synthesis by coupling amino acids. Experimental studies suggest that hydrogen sulfide (H 2 S), a precursor of OCS, could be present in astrophysical environments. In the present study, we used a microwave-discharge hydrogen-flow lamp, simulating the interstellar UV field, and a monochromatic synchrotron light beam to irradiate CO:H 2 S and CO 2 :H 2 S ice mixtures at 14 K with vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) photons in order to study the effect of the photon energy and carbon source on the formation mechanisms and production yields of S-containing products (CS 2 , OCS, SO 2 , etc.). Results show that (1) the photo-induced OCS production efficiency in CO:H 2 S ice mixtures is higher than that of CO 2 :H 2 S ice mixtures; (2) a lower concentration of H 2 S enhances the production efficiency of OCS in both ice mixtures; and (3) the formation pathways of CS 2 differ significantly upon VUV and EUV irradiations. Furthermore, CS 2 was produced only after VUV photoprocessing of CO:H 2 S ices, while the VUV-induced production of SO 2 occurred only in CO 2 :H 2 S ice mixtures. More generally, the production yields of OCS, H 2 S 2 , and CS 2 were studied as a function of the irradiation photon energy. Heavy S-bearing compounds were also observed using mass spectrometry during the warm-up of VUV/EUV-irradiated CO:H 2 S ice mixtures. The presence of S-polymers in dust grains may account for the missing sulfur in dense clouds and circumstellar environments.

Infrared Spectra and Interstellar Sulfur: New Laboratory Results for H2S and Four Malodorous Thiol Ices

“…Quantitative descriptions of the IR spectrum of amorphous H 2 S are surprisingly scarce as shown by tracing the origin of the currently used A′(S-H) band strength. A recent astrochemical paper on sulfur chemistry (Chen et al 2015) used A′(S-H)= 2.0×10 −17 cm molecule −1 for amorphous H 2 S at 14 K, taken from a paper of Jiménez-Escobar & Muñoz Caro (2011). The latter authors derived this value by rescaling one from Smith (1991) of A′(S-H)=2.9×10 −17 cm molecule −1 for crystalline H 2 S at 88 K to amorphous H 2 S at 10 K. Smith, in turn, derived his A′ for amorphous H 2 S by combining all of the band intensities of Ferraro et al (1980) for crystalline H 2 S in the S-H stretching region.…”

Electron Irradiation and Thermal Processing of Mixed-Ices of Potential Relevance to Jupiter Trojan Asteroids

“…This band is assigned to the C=O stretching mode of OCS, which is observed at 2019 cm −1 in pure OCS ice and shifted to 2046 cm −1 in OCS-H 2 O and OCS-CO 2 mixtures (Ferrante et al 2008). Previous experimental studies reported the formation of OCS after irradiation of ices containing CO or CO 2 as the carbon source and H 2 S or SO 2 as sulfur source (Ferrante et al 2008;Chen et al 2015). Our assignment of the 2040 cm −1 band to OCS is supported by the absence of this band in the infrared spectrum of the irradiated "without H 2 S" mixture, where no sulfur was present.…”

Sulphur-bearing species in the coma of comet 67P/Churyumov–Gerasimenko

“…The products depend on the initial ice composition, as illustrated in recent experiments (e.g. Garozzo et al 2010;Chen et al 2015). Note that OCS can potentially also be formed by low-temperature surface reactions of CO with S without the need for photolysis or radiolysis.…”