Icy dust grains in the interstellar medium : their properties and impact
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The behaviour of atoms and molecules on interstellar dust grain analogue surfaces has been probed with surface science techniques such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Chemical systems under investigation include O atoms, O2, CO, H2O and N2O as deposited either on a silica (SiO2) surface representative of an interstellar dust grain or on H2O surfaces. As H2O is the dominant solid state chemical species in the interstellar medium (ISM) preliminary experiments have been conducted to investigate one of its formation pathways. Such experiments involved bombarding SiO2 or H2O surfaces with O atoms, producing O2 and O3 molecules as determined by RAIRS and TPD. During the initial stages of H2O growth on dust grains in the ISM, only small quantities will be found in the solid state. The de-wetting behaviour of such quantities was investigated by directly dosing H2O onto the SiO2 surface and observing the νOH vibrational band with time and temperature. Through such experiments, H2O has been observed to de-wet even at cryogenic temperatures of 17 K with an activation energy of about 2 kJ mol-1. The effect of this in the ISM is that bare dust grain surfaces will be left exposed for other molecules, such as CO, to interact with. Such interactions may be stronger than those of the molecules directly interacting with a water surface, meaning molecules will reside in the solid state for longer times. As icy mantles develop throughout the lifetime of a molecular cloud, adsorbed CO becomes a large part of the icy mantle. Multilayers of CO were investigated and shown to exhibit a bulk potential. This charge is due to a spontaneous electric (spontelectric) field arising as certain dipolar molecules align in the solid state. To investigate the spontelectric effect in CO, N2O was first examined and shows a correlation between IR features and the direct measurement of the spontelectric potential of N2O. Such a link was found through the temperature dependent shifts in the LO-TO splitting of the N2O. CO exhibits the same temperature dependence and spontelectric parameters have been extracted to show that a spontelectric potential of 6.7 mV per monolayer is created as multilayers adsorb on a surface. The effect of this in the ISM would be to reduce the gas-phase charge and ionisation fractions in molecular clouds of the ISM which in itself has the potential to have wide-reaching implications.