A laboratory study of ion-induced erosion of ice-covered carbon grains

Abstract

Context. It has been confirmed that solid carbon dioxide (CO2) is abundantly present along the line of sight to quiescent clouds and star-forming regions via space IR observations with ISO-SWS and Spitzer Space Telescope. Since CO2 has low abundance in the gas-phase, the assumption is that it is synthesized on grains after energetic processing of icy mantles and surface reactions. Aims. The role of solid carbon is investigated as a reservoir for molecule formation and structural modifications of the material with and without an ice layer upon ion bombardment. Methods. A gas-phase condensation technique was used to prepare a layer of 13C amorphous grains. These grains were covered with H2O and O2 ice and finally bombarded with 200 keV protons. The formation of new molecular species was analyzed using IR spectroscopy. The formation cross sections of solid 13CO and 13CO2 were determined from the increase in the column density as a function of the fluence. In addition, bare carbon grains were bombarded with a comparable fluence of protons to study the processing of the grains without ice layer. Imaging techniques such as transmission electron microscopy were used to monitor the changes in the structure. Results. CO and CO2 were formed efficiently at the interface between ice and solid carbon grains at the expense of solid carbon, leading to strong grain erosion. Given the initial thickness of our C-samples (about 120nm), this resulted in an erosion of about 50% after 200 keV proton bombardment with 6.76 × 1016 ions/cm2. The column density of CO and CO2 follows an exponential trend as a function of the irradiation fluence. The asymptotic values obtained when O2 ice is deposited on top of the carbon grains are about one order of magnitude higher than the values obtained when H2O ice is deposited on the solid carbon layer. The carbon grains were strongly graphitized upon ion bombardment in a surface layer. Less graphitization accompanied by the formation of fullerene molecules and structures from cage fragments present in the original material were observed beneath the graphitic layer. Conclusions. The formation of CO and CO2 at the expense of solid carbon strongly restricts the lifetime of the solid carbon material and may influence the formation of more complex molecules in astrophysical environments. Graphitization of carbonaceous grains upon ion bombardment affect the spectral properties of the carbon grains in particular in the far-IR range.