Abstract
Molecules possessing NO bonds are important precursors in biology, however in the interstellar medium, such NO bond containing molecules are observed in relatively low abundance. In the present study we focus on explaining the abundances of NO, HNO and NOH in the interstellar medium. As a number of reactions in space occur on water-ice surfaces, it is hypothesised that the hydrogenated species might remain adsorbed to the surface once formed, thus limiting their detection in the gas-phase by rotational spectroscopy techniques currently employed. This hypothesis is tested in the present work to see whether HNO and NOH are likely to be bound to the surface. Here, a combination of complementary computational and experimental techniques have been employed to explore this chemistry.
Initially, the computational aspect of this project involved benchmarking against previous studies to establish suitable water-ice models. A systematic study of adsorption of NO, HNO and NOH on hexagonal ice surfaces then provided an insight into the influence that the local water-ice environments had on the binding energies. Consistent with interstellar observations, NO was found to bind the weakest. Both hydrogenation species formed stronger bonds with the surface, the HNO binding energies intermediate of NO and NOH.
The reaction paths connecting NO and the respective hydrogenated species were then investigated. Here, both hydrogenation reactions on the hexagonal ice surface were found to be barrierless, suggesting facile formation of HNO and NOH. Interestingly, a much stronger thermodynamic driving force was observed in the HNO example, hence, it was concluded that HNO is likely to be the predominant species formed via NO hydrogenation on hexagonal ice surfaces.
In order to provide the experimental aspect of this project with spectroscopic probes, the vibrational frequencies of both the free and adsorbed molecules were calculated. Replicating interstellar medium conditions in the laboratory, thin water-ice films were deposited at low temperatures and under high vacuum. NO was then leaked into the vacuum chamber and the system irradiated with UV for up to 90 hours to generate hydrogen atoms; in this way simulating UV processing of interstellar medium grains. This entire process was monitored using vibrational spectroscopy. Interestingly, the experiments revealed a more complicated reaction network than the simple conversion of NO to HNO, or NOH, with a number of NxOy species being observed. However, temporal changes in peak profiles provided evidence that active photochemistry was occurring.
A final branch of calculations investigated the binding energies and reaction energetics of the different species on an amorphous water-ice surface. These calculations were completed as amorphous ice is the predominant form of water-ice observed in space. Here, the goal was to determine whether the hexagonal ice models (used in both the computational and experimental aspects of this project) provided a reasonable proxy for the amorphous water-ice surface. Similar trends were observed for the relative binding energies of NO, HNO and NOH on both types of ice. However, in contrast to the hexagonal ice surface, a barrier was observed in both cases going from NO to the hydrogenated product. Further work is required to investigate how processes such as tunnelling and hydrogen diffusion might influence this barrier to hydrogenation on the amorphous ice surface.