Energetic electron irradiations of amorphous and crystalline sulphur-bearing astrochemical ices

Abstract

Laboratory experiments have confirmed that the radiolytic decay rate of astrochemical ice analogues is dependent upon the solid phase of the target ice, with some crystalline molecular ices being more radio-resistant than their amorphous counterparts. The degree of radio-resistance exhibited by crystalline ice phases is dependent upon the nature, strength, and extent of the intermolecular interactions that characterise their solid structure. For example, it has been shown that crystalline CH3OH decays at a significantly slower rate when irradiated by 2 keV electrons at 20 K than does the amorphous phase due to the stabilising effect imparted by the presence of an extensive array of strong hydrogen bonds. These results have important consequences for the astrochemistry of interstellar ices and outer Solar System bodies, as they imply that the chemical products arising from the irradiation of amorphous ices (which may include prebiotic molecules relevant to biology) should be more abundant than those arising from similar irradiations of crystalline phases. In this present study, we have extended our work on this subject by performing comparative energetic electron irradiations of the amorphous and crystalline phases of the sulphur-bearing molecules H2S and SO2 at 20 K. We have found evidence for phase-dependent chemistry in both these species, with the radiation-induced exponential decay of amorphous H2S being more rapid than that of the crystalline phase, similar to the effect that has been previously observed for CH3OH. For SO2, two fluence regimes are apparent: a low-fluence regime in which the crystalline ice exhibits a rapid exponential decay while the amorphous ice possibly resists decay, and a high-fluence regime in which both phases undergo slow exponential-like decays. We have discussed our results in the contexts of interstellar and Solar System ice astrochemistry and the formation of sulphur allotropes and residues in these settings.

Interaction of H2S with H atoms on grain surfaces under molecular cloud conditions

“…The (so far) nondetection of solid-phase H 2 S in interstellar clouds thus poses a question regarding the fate of H 2 S in interstellar icy mantles. One likely explanation for its absence in observations is that solid-phase H 2 S is effectively destroyed by, for instance, energetic processing, which is known to result in solid-phase sulfur chemistry (e.g., Moore et al 2007;Garozzo et al 2010;Jiménez-Escobar & Muñoz Caro 2011;Jiménez-Escobar et al 2014;Chen et al 2015;Shingledecker et al 2020;Cazaux et al 2022;Mifsud et al 2022). In fact, the photochemistry of H 2 S induced by UV photons has been suggested as a potential sulfur sink, as it has been shown to produce allotropic forms of S (S n ) that are largely refractory (especially for n > 4).…”

“…by quantum tunneling through an effective barrier of ∼1500 K (Lamberts & Kästner 2017). The HS radical can subsequently be hydrogenated to reform H 2 S. Alternatively, H 2 S can also be energetically processed to form species such as H 2 S 2 and a wide range of S allotropes (Moore et al 2007;Garozzo et al 2010;Jiménez-Escobar & Muñoz Caro 2011;Jiménez-Escobar et al 2014;Chen et al 2015;Shingledecker et al 2020;Cazaux et al 2022;Mifsud et al 2022).…”

Laboratory infrared spectra and fragmentation chemistry of sulfur allotropes

“…Sulfur allotropes have been proposed as an important sulfur reservoir in molecular clouds 18 , formed, for example, upon UV irradiation of H 2 S ices 19 , or electron irradiation of H 2 S and SO 2 ices 20 . In the latter laboratory study, an apparent depletion of accountable sulfur budget was observed, attributed to the possible formation of sulfur allotropes.…”

A systematic mid-infrared spectroscopic study of thermally processed SO2 ices

“…11,12 Indeed, previous experimental work has suggested that the irradiative processing of SO 2 ice using 1.5 keV electrons results in a relatively facile conversion to infrared-inactive molecular forms of sulphur. 13 The detection of SO 2 on various Solar System bodies has also proven to be somewhat contentious, due to these observations primarily relying on ultraviolet-visible spectroscopy, which is more ambiguous than infrared spectroscopy in categorically assigning absorption features to particular molecular species. For example, Hendrix et al 14 suggested that the observed ultraviolet-visible reflectance spectra of the dwarf planet Ceres between 320-400 nm could be approximated fairly well by laboratory spectra of phyllosilicate minerals (such as montmorillonite) intermixed with solid SO 2 and allotropic forms of elemental sulphur.…”

“…11,12 Indeed, previous experimental work has suggested that the irradiative processing of SO 2 ice using 1.5 keV electrons results in a relatively facile conversion to infrared-inactive molecular forms of sulphur. 13…”