Laboratory investigations of the thermal and non-thermal processing of condensed aromatic hydrocarbons in the interstellar medium

Abstract

The thermal and non-thermal desorption of C6H6 has been investigated as a model for the behaviour of other aromatic hydrocarbons existing in the condensed phase in the interstellar medium. An interstellar dust grain mimic based on amorphous SiO2, to represent the interstellar silicate grain population, has been developed for use as a substrate in these experiments. Temperature programmed desorption experiments reveal a broad distribution of binding sites on this surface, with C6H6 desorbing thermally over a wide temperature range. The desorption from compact amorphous solid water displays simpler desorption kinetics with evidence for the formation of C6H6 islands on the water surface, demonstrating the importance of using realistic interstellar grain mimics in experiments probing surface sensitive interstellar processes. Kinetic parameters have been obtained for these systems, along with those for thicker multilayer films of ice. Photon irradiation of C6H6 / H2O layered ice systems at 250 nm results in the desorption of both species as observed using time-of-flight mass spectrometry. The molecules desorb with high translational energies which would represent a significant energy injection into the cold interstellar gas phase. Three desorption processes, desorption via direct adsorbate-, indirect adsorbate- and substrate-mediated desorption, are proposed for the observed desorption profiles. The desorption of H2O relies on energy transfer following photon absorption by a C6H6 molecule bound to a surface (H2O)n cluster, which results in the unimolecular decomposition of the complex. Kinetic simulations indicate that such processes may lead to an enhancement of photon-induced desorption at the edges of dense interstellar clouds. Experiments have also been performed to study the electron-stimulated desorption of molecules from C6H6 adsorbed on top of a water ice film. A highly efficient desorption channel with a cross-section in excess of 10-15 cm2 is in operation for low coverages of C6H6 and is attributed to the migration of excitons formed within the bulk of the H2O ice to the vacuum interface. A slower desorption component was also observed, which is attributed to a diffusion limited desorption step. These observations imply that electron stimulated desorption is likely to be an important non-thermal desorption process within dense clouds. No evidence for any chemical reaction products was observed through IR spectroscopy.