de Souza Bonfim Víctor, Barbosa de Castilho Roberto, Baptista Leonardo, Pilling Sergio
Universidade do Vale do Paraíba - UNIVAP/Laboratorio de Astroquimica e Astrobioloiga - LASA, São José dos Campos, SP, Brazil.
Phys Chem Chem Phys. 2017 Oct 11;19(39):26906-26917. doi: 10.1039/c7cp03679e.
In this combined experimental-theoretical work we focus on the physical and chemical changes induced by soft X-rays on sulfur dioxide (SO) ice at a very low temperature, in an attempt to clarify and quantify its survival and chemical changes in some astrophysical environments. SO 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 SO 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 (SO) and formation (SO) cross sections were 2.5 × 10 cm and 2.1 × 10 cm, respectively. The chemical equilibrium (88% of SO and 12% of SO) was reached after the fluence of 1.6 × 10 photons cm. The SO 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) in simulated SO ice: (i) SO + O → SO, (ii) SO + O → SO, and (iii) SO + O → SO + e → SO. The amorphous solid environment effect decreases the reactivity of intermediate species towards SO 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.
在这项结合实验与理论的工作中，我们聚焦于极低温下软X射线对二氧化硫（SO）冰所引发的物理和化学变化，旨在阐明并量化其在某些天体物理环境中的存活情况及化学变化。SO是木星一些卫星的重要组成部分，在原恒星周围的冰中也有观测到。测量是在巴西坎皮纳斯的巴西同步辐射光源（LNLS/CNPEM）进行的。SO冰样品（12K）暴露于主要为软X射线（6 - 2000eV）的宽带光束下，并通过红外光谱进行原位分析。X射线光解吸产率（上限）约为每光子0.25个分子。确定的有效破坏（SO）和形成（SO）截面值分别为2.5×10 cm和2.1×10 cm。在1.6×10光子/cm的注量后达到化学平衡（88%的SO和12%的SO）。在二级Møller - Plesset微扰理论（MP2）水平上研究了SO的形成通道，该理论显示了模拟SO冰中三条最有利的反应路径（ΔH < -79 kcal/mol）：（i）SO + O → SO，（ii）SO + O → SO，以及（iii）SO + O → SO + e → SO。非晶态固体环境效应降低了中间物种对SO形成的反应活性，离子物种受到的影响更大。这项工作中实验确定的有效截面和理论反应通道使我们能够更好地理解某些富硫天体物理环境的化学演化。
