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.

Keywords: amorphous ice; astrochemistry; crystalline ice; electron irradiation; planetary science; radiation chemistry; sulphur.

FIGURE 1

Top-view schematic diagram of the ICA set-up. Note that electron irradiations are carried out such that projectile electrons impact the target ices at 36° to the normal. Figure reproduced from Mifsud et al. (2021b) with the kind permission of the European Physical Journal (EPJ).

FIGURE 2

FT-IR spectra of the amorphous and crystalline phases of H₂S and SO₂ ices at several points during their irradiation by energetic electrons at 20 K. Note that the fine structures coincident with the SO₃ absorption band in the spectrum of the electron irradiated amorphous SO₂ ice are caused by instabilities in the purge of the detector. Moreover, the initial increase in the intensity of the amorphous SO₂ asymmetric stretching mode is likely caused by the radiation-induced compaction of the porous ice.

FIGURE 3

Decay of amorphous and crystalline H₂S column densities normalised to the initially deposited column density during irradiation using 2 keV electrons. Note that the average decay trends are fitted by two exponential decay functions joined at a fluence of 1.4 × 10¹⁵ electrons cm⁻².

FIGURE 4

Decay of amorphous and crystalline SO₂ column densities normalised to the initially deposited column density during irradiation using 1.5 keV electrons. Note that the average decay trends are not fits and are plotted solely to guide the eye.

FIGURE 5

Above: Column density of H₂S₂ from amorphous and crystalline H₂S ices irradiated using 2 keV electrons at 20 K. Below: Column density of SO₃ from amorphous and crystalline SO₂ ices irradiated using 1.5 keV electrons at 20 K. Column densities have been normalised to the initially deposited column density of the parent molecular ice. Note that in the case of H₂S₂ the average trends are fitted by logarithmic functions while in the case of SO₃ the average trends are not fits and are plotted solely to guide the eye.

FIGURE 6

Sulphur budgets of the electron irradiated amorphous and crystalline H₂S and SO₂ ices considered in this study. Uncertainties in the normalised abundance of the parent and primary product molecules are estimated to be within 3%. The quantity of unobserved sulphur represents an upper bound for the abundance of atomic or allotropic sulphur formed as a result of irradiation, since it is not known how many (if any) sulphur-containing species were sputtered or desorbed from the bulk ice. Note that the notations “a-” and “c-” used in the caption indicate whether the irradiated ice is amorphous or crystalline.