Photochemical Effects on the Ices and Dust Grains of Astronomical Importance

Dust grains and the ice layers condensed on the grains in the interstellar medium are important places for new molecules to form. This thesis aims to use laboratory experimental methods to investigate Lyman-α radiation (10.2 eV) induced photochemical effects that occur on astronomical ices and dust grains. We perform experiments in an ultrahigh vacuum cell and use cryogenic cooling to mimic the interstellar environment. The reaction processes and products are studied by Fourier Transform Infrared Spectroscopy (FTIR), Quadrupole Mass Spectrometry (QMS), and other surface analysis methods.
We conducted a comprehensive study on the photochemical effects during the life cycle of interstellar CO2 ice, from its formation pathways to its photodissociation and photodesorption mechanisms. A new Eley-Rideal type mechanism of CO + OH reaction is proposed to explain the CO2 ice formation in the ISM. This reaction process describes an incoming gas phase CO molecule directly react with a surface OH radical produced by photodissociation of H2O ice. We also studied the photodissociation of CO2 ice, and found an isotope effect which favors the light isotopomer, 12CO2, to dissociate faster than 13CO2, due to the difference in the probability of dissociating the C-O bond in electronically-excited CO2* molecules when in close contact with the ice matrix. In the studies of photodesorption of CO2 ice, we demonstrated the efficiency of CO2 desorption is strongly correlated inversely to the efficiency of CO2 vibrational relaxation in the ice. In addition, we show how the interaction between high energy photons and a CO2 ice lattice affects the photodesorption efficiency. Besides ices, we studied the UV-induced surface reactions of N2O molecules adsorbed on SiO2 grain surfaces. The grain surface is shown to favor association reactions in producing NO2 and N2O4, comparing to those reactions in the gas phase. The roles of surface functional groups and their effects on the reaction rates, such as cage effect, quenching and spacing effects are also studied.