Astrochemical Surface Science: Water Desorption from Nanostructured Graphite

Licentiate thesis, 2013

To date, around 180 different molecules have been identified in the interstellar medium (ISM). Some of them form molecular ices on cold interstellar dust grain surfaces. An interstellar dust grain particle is in the order of nanometres in size and is usually composed of a carbonaceous or silicatious grain core. Surface processes on dust grains are important for formation of molecules, as well as stars and planets. To aid understanding of theses processes, it is advantageous to study model systems. In laboratory experiments, interstellar dust grains are commonly modelled by flat metal surfaces although such structures are far from reality. With the motivation to bridge this gap, in this thesis, nanostructured graphite is used to model grain core surfaces in order to study the thermal desorption of water, the main component in the mixture of interstellar ices. The nanostructured graphite surface does not only exhibit a rich surface morphology but also a characteristic absorption in the visible range of the electromagnetic spectrum. The thermal desorption of water is studied by temperature programmed desorption (TPD) in combination with quadrupole mass spectrometry (QMS) and an optical reflection technique. A key question addressed is how the kinetics and energetics of water desorption are influenced by the surface morphology for different water ice coverages. It is found that the nanostructured graphite surface gives rise to the presence of multiple peaks in TPD-QMS spectra that are assigned to water bound in two-and three-dimensional hydrogen-bonded networks, defect-bound water, and to water intercalated in the graphite structures. The characteristic absorption of the nanostructured graphite surface is utilised for a sensitive optical detection to follow water desorption. With this technique, the intensity of the diffusely reflected light from the surface is measured during a temperature ramp. The optical signal is found to be dependent on the water ice thickness and structure. It is also possible to observe the water ice in subsurface regions.