SO₃ formation from the X-ray photolysis of SO₂ astrophysical ice analogues: FTIR spectroscopy and thermodynamic investigations

Abstract

In this combined experimental-theoretical work we focus on the physical and chemical changes induced by soft X-rays on sulfur dioxide (SO₂) ice at a very low temperature, in an attempt to clarify and quantify its survival and chemical changes in some astrophysical environments. SO₂ is an important constituent of some Jupiter moons and has also been observed in ices around protostars. The measurements were performed at the Brazilian Synchrotron Light Source (LNLS/CNPEM), in Campinas, Brazil. The SO₂ ice sample (12 K) was exposed to a broadband beam of mainly soft X-rays (6-2000 eV) and in situ analyses were performed by IR spectroscopy. The X-ray photodesorption yield (upper limit) was around 0.25 molecules per photon. The values determined for the effective destruction (SO₂) and formation (SO₃) cross sections were 2.5 × 10^-18 cm² and 2.1 × 10^-18 cm², respectively. The chemical equilibrium (88% of SO₂ and 12% of SO₃) was reached after the fluence of 1.6 × 10¹⁸ photons cm^-2. The SO₃ formation channels were studied at the second-order Møller-Plesset perturbation theory (MP2) level, which showed the three most favorable reaction routes (ΔH < -79 kcal mol^-1) in simulated SO₂ ice: (i) SO + O₂ → SO₃, (ii) SO₂ + O → SO₃, and (iii) SO₂ + O⁺ → SO₃⁺ + e^- → SO₃. The amorphous solid environment effect decreases the reactivity of intermediate species towards SO₃ formation, and ionic species are even more affected. The experimentally determined effective cross sections and theoretical reaction channels identified in this work allow us to better understand the chemical evolution of certain sulfur-rich astrophysical environments.