Photochemistry of carbon monoxide and methanol in water and nitric acid hydrate ices: A NEXAFS study

Abstract

Soft X-ray induced chemistry of H₂O, CO and CH₃OH and the effects of the water and nitric acid hydrate (HNO₃·1.65H₂O) matrix on the photochemistry of CO and CH₃OH have been investigated using NEXAFS spectroscopy. For pure H₂O, CO and CH₃OH ices, we show that the destruction rates are strongly limited by back reactions, leading to strikingly high survival rates of these molecules upon the harsh irradiation conditions to which they are submitted. We also evidence the interplay between the photochemical reactions of CO and CH₃OH and those of the matrix. The OH and O radicals released by the photolysis of H₂O and HNO₃ react with the CO and CH₃OH and their fragments, considerably reducing the survival rates compared to pure CO and pure CH₃OH ices, especially in presence of nitric acid, and dramatically enhancing the formation of CO₂ at the expense of CO. Because NEXAFS spectroscopy allows identifying which reactions are important among those possible, it emerges a simple picture of the photochemical routes of CO and CH₃OH in the H₂O and HNO₃/H₂O environments.