Wolff, A.J.; (2006) Adsorption and desorption of mixed molecular ices from a cosmic dust grain analogue surface. Doctoral thesis , University of London.

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Abstract

Surface science is playing an ever more prominent role in the field of astronomy. More than 220 different molecules have so far been observed in the interstellar medium (ISM), and for several of these molecules, the observed abundance is such that the molecules cannot be formed by gas phase reactions alone. Astronomers have proposed that they are instead formed by heterogeneous reactions that take place on the surface of dust grains. The two alcohols methanol and ethanol are just two of the molecules typically observed in both the gas and solid phase in the ISM. In the solid phase, they are found frozen out with the more abundant water, as molecular ices on the surface of dust grains. Both alcohols can be viewed as evolutionary indicators in the vicinity of hot cores. Hot cores are compact objects found in close to newly formed massive stars they are dense and relatively warm and show atypical gas-phase molecular compositions. The gas-phase composition, and therefore the evolutionary stage of the hot core, can be understood by considering the sublimation behaviour of molecular ices on the dust grains within the molecular cloud. This thesis presents the results of investigations on the adsorption and desorption of methanol and ethanol in both the pure state and in combination with water. In each case the deposition occurs on a highly oriented pyrolytic graphite (HOPG) surface. HOPG is considered to be a suitable interstellar dust grain analogue, as dust grains in the ISM are composed of mainly carbonaceous and silicaceous material. Temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) studies of methanol and ethanol ices, mixed with water, are presented. The adsorption and desorption of each species deposited on a layer of amorphous solid water ice is compared to those of codeposited ice layers. In all systems, there is evidence for molecular adsorption in a physisorbed state and for interactions between the investigated molecule and the water ice. As expected, the sequentially deposited systems show different behaviour when compared with the codeposited systems. The information obtained from these experiments is of direct relevance to astronomers, as the measured desorption energies can be incorporated directly into astronomical models. This, in turn, helps to lead to a greater understanding of star formation, and hence of the Universe in which we live.

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