Edridge, J.L.; (2010) Adsorption and desorption of model interstellar ices on a dust grain analogue surface. Doctoral thesis , UCL (University College London).

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Abstract

Molecular ices freeze out on the surface of dust grains within the interstellar medium (ISM). Dust grains form 1% of the mass of the ISM, and are comprised of siliceous or carbonaceous material. Molecular ices are H2O-rich, with major components including CO and CO2, along with trace amounts of sulphur containing molecules, including SO2. CS2 ice has also been detected in cometary comae. Previous studies of the adsorption and desorption of H2O-rich ices show that the H2O structure dictates the desorption, diffusion and trapping of molecules within the ice. This is important in star forming regions, where the evaporation of molecular ices is not spontaneous. Hence, the chemical composition of the resultant star or planetary system is affected by these processes in molecular ices. There is little previous experimental work concerning the adsorption and desorption of sulphur bearing ices. Therefore a study of CO2, SO2 and CS2-bearing, H2O-rich, ices adsorbed on a carbonaceous surface at ≤ 33 K, has been conducted using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Qualitative inspection of the ices shows that the molecular size impacts the ability of a species to diffuse or trap within the ice. Quantitative analysis of the TPD spectra has also been conducted, to determine desorption energies and orders. These have been used in an astrochemical model, simulating the warming of ices in a star forming region. A model has also been created to simulate RAIR spectra of adsorbate layers on a surface. Experiments show that amorphous CO and CO2 ices exhibit unusual splitting of the C-O stretch, typically characteristic of crystalline ice. A model was developed to investigate this splitting within the amorphous ice. Results show that this splitting occurs as a function of the dielectric properties of the ice.

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