Paper SS1-WeA2
Photon- and Electron-stimulated Desorption from Laboratory Models of Interstellar Ice Grains

Wednesday, November 11, 2009, 2:20 pm, Room M

Dense molecular clouds, in our own and other galaxies throughout the Universe, are the seats of evolution in the present day Universe. New stars and their planetary systems are born in such environments from the chemically rich soup of gas and icy dust from which these environments are made. In the last decade or so, astronomers have turned to the surface science community to help explain the role played by physical and chemical processes in coupling of the gas and solid (dust grain) phases in these crucially important regions of space.

We have recently been investigating one key aspect of that gas-grain interaction; the role played by non-thermal (photon- and low energy electron-driven) processes in returning components of the icy grain mantle to the gaseous interstellar medium. Ultrathin layers of predominantly water (H₂O) ice grow reactively on dust grains; such layers can accumulate adsorbed volatiles such as carbon monoxide (CO) from the gaseous interstellar medium and are observed with sufficient ease in the infrared to be mapped. Indeed such icy mixtures are firmly believed to be the home of chemical evolution in these dense environments as the simple ices (H₂O, CO, NH₃ etc.) are converted by a combination of thermal and non-thermal processes into more complex species (e.g. CO₂, CH₃OH, CH₃NH₂ etc.). Further processing of these complex icy mixtures may lead to the formation of prototypical molecules which may, following exogenous delivery to a nascent planet, provide the raw materials for biological evolution. However such a drive towards increasing chemical complexity takes time; time in which competition from non-thermal desorption processes may in fact remove the icy mantle from its substrate exposing the fragile molecular contents to the harsher environment of the gaseous interstellar medium. We describe preliminary studies of both photon- and low energy electron-driven desorption from a model interstellar ice deposited on model grain surfaces under UHV conditions. We will report on the rates of desorption mediated by photon- and electron-stimulated interactions and, where known, outline the dynamical consequences of such desorption.