Desorption of neutrals, cations, and anions from core-excited amorphous solid water

Abstract

Core-excitation of water ice releases many different molecules and ions in the gas phase. Studying these desorbed species and the underlying mechanisms can provide useful information on the effects of X-ray irradiation in ice. We report a detailed study of the X-ray induced desorption of a number of neutral, cationic and anionic species from amorphous solid water. We discuss the desorption mechanisms, and the relative contributions of Auger and secondary electrons (X-ray induced Electron Stimulated Desorption) and initial excitation (direct desorption) as well as the role of photochemistry. Anions are shown to desorb not just through processes linked with secondary electrons but also through direct dissociation of the core-excited molecule. The desorption spectra of oxygen ions (O + , OH + , H2O + , O − , OH − ) give a new perspective on their previously reported very low desorption yields for most types of irradiation of water, showing that they mostly originate from the dissociation of photoproducts such as H2O2.

X-ray induced desorption and photochemistry in CO ice

“…The core electronic state is thus remarkably unaffected by the condensed environment. This stands in contrast with what is observed in the case of water ice 21,35 , highlighting the differences between hydrogen-bond solids and weakly interacting Van der Waals solids. In the right inset of the upper panel in fig.…”

“…Experiments were performed in the SPICES 2 set-up. Some aspects of the experiments have already been detailed elsewhere 20,21 . Briefly, the set-up is an ultra-high vacuum chamber equipped with a closed-cycle helium cryostat, reaching a base temperature of 15 K at the sample holder and a base pressure of ∼ 1 × 10 −10 mbar at 15 K. The substrate used was a technical copper surface (polycrystalline OFHC copper), electrically insulated from the holder by a kapton foil.…”

“…Absorption spectroscopy of the ice is performed by recording the current of the electrically insulated substrate that appears upon irradiation from the ejection of electrons from the surface (total electron yield, TEY), and scanning the photon energy. The absolute energy scale was set in a way described in a previous paper 21 . The position of the main resonance peak of solid CO is known to be the same as in the gas phase 29 and thus served as a cross-check of the validity of our energy scale.…”

“…Cosmic-ray induced desorption from CO has also been studied from early on 17,18 and more recently with heavy ions 19 . In several recent papers, we have explored the a Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, F-75005, Paris, France; E-mail: dupuy@fhi.mpg.de b Institut de Chimie Physique, UMR 8000, CNRS, Université Paris-Saclay, 91405, Orsay, France c Laboratori Nazionali di Frascati (LNF)-INFN I-00044 Frascati d CERN, CH-1211 Geneva 23, Switzerland astrophysical implications 20 and the details 21 of the X-ray photodesorption process from water ice, as well as X-ray photodesorption from methanol in pure 22 and mixed ices 23 . Other authors also studied X-ray photodesorption from mixed ices 24,25 .…”

“…Through the study of water ice 20 , we found that X-ray photodesorption is a potentially important process in some regions of the interstellar medium, comforting the need for quantitative studies on other relevant molecular ices. Each of the many desorbing species (neutrals, cations and anions) from water ice exhibited different behaviours, leading to a variety of possible desorption mechanisms and giving insights into the details of energy relaxation of X-ray excited water ice 21 . A similarly complete survey of desorbing species from CO ice is therefore also of interest.…”