Date of Award
Doctor of Philosophy (PhD)
cosmic dust, interstellar medium, oxygen, surface kinetics
Physical Sciences and Mathematics
The formation of molecules in the interstellar medium (ISM) takes place both in the gas phase and on surfaces of cosmic dust grains. Gas phase reactions alone are found to be insufficient to account for the observed abundance of molecules such as molecular hydrogen, water, carbon dioxide, methanol as well as many other complex molecules; grain surfaces must be involved as catalysts to explain their formation. In this thesis we study the physical and chemical processes on surfaces of cosmic dust grain analogues in simulated ISM environments. Oxygen is the third most abundant element in the universe and is present in many astrobiologically important molecules. The desorption energy of atomic oxygen is a fundamental parameter that enters ISM models because it controls the residence time of this atom on a surface. However, it has not been measured in the laboratory. In this thesis, this parameter is measured by using both an indirect and a direct method. The measured value agrees with model predictions based on astronomical observations. The formation of two oxygen-containing molecules, water and hydroxylamine, is studied next. Water is the main component of ice mantles in dense clouds and is indispensable for the origin of life. Its formation via ozone hydrogenation on an analog of a warm dust grain is studied experimentally. The desorption energy of an important intermediate product in the reaction, the OH radical, is also inferred. Hydroxylamine is a precursor to the formation of glycine, which is the simplest amino acid. The formation of hydroxylamine via the oxidation of ammonia is studied by sequential deposition of ammonia and atomic oxygen and is followed by temperature programmed desorption experiments. The measured high reaction efficiency predicts that ammonia oxidation on grain surfaces could be an important route to hydroxylamine formation. The last chapter of this thesis introduces a rate equation model to simulate surface kinetics, including diffusion and desorption, of atoms and molecules on non-uniform surfaces.
He, Jiao, "Laboratory Studies of Gas-Grain Processes on Cosmic Dust Analogues" (2014). Dissertations - ALL. 195.