Towards understanding the formation of water on intersteller dust grains

Abstract

Although several research groups have studied the formation of H2, both experimentally and computationally, few have explored the surface formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface and, over time, lead to the formation of icy mantles coating interstellar dust grains. The most abundant of these species within the ice is H2O. The first half of this thesis introduces the construction and characterisation of the new dual atomic beam apparatus built to explore the surface formation mechanism of H2O. The apparatus has been designed to enable singular or dual atomic or molecular oxygen and hydrogen beams to be adsorbed onto a range of astronomically relevant substrates. Analysis of the surface chemistry can be performed using a combination of temperature programmed desorption, molecular beam modulation spectrometry, quartz crystal microgravimetry and reflection-adsorption infrared spectroscopy techniques. The remainder of this thesis discusses the results obtained by performing temperature programmed desorption experiments. Kinetic analysis was deduced for: H2O on bare silica; O2 on bare silica; O2 on compact amorphous solid water on silica; and O2 on porous amorphous solid water on silica. The results obtained were used towards constructing a simulation model mimicking the desorption of O2 from the icy mantles of interstellar dust grains under dense molecular cloud environments. The analysis revealed that sub-monolayer coverages of O2 followed first order desorption kinetics with a range of desorption activation energies from all of the surfaces studied. Multilayer coverages of O2 from silica were unexpectedly found to follow fraction order kinetics. Further experiments were performed to explore the origins of this multilayer fractional desorption order. The results obtained revealed that the kinetic order decreased roughly by half as the species change from O2 to CO to N2 suggesting the underlying amorphous silica surface appeared to be the cause for this unusual observation. Preliminary atomic O beam TPD experiments had also been performed from a range of astronomical relevant surfaces. The initial results indicated that O2, O3, H2O2 and 13CO2 were formed on the surface. However, the exact surface formation mechanism could not be concluded from these single experiments.