Laboratory Constraints on Thermal and Photon-Induced Processes in Interstellar Ices

Chemistry in icy mantles condensed on the surface of dust grains is an important route to the molecular complexity observed in the interstellar medium. The molecules that are produced in the ice may be transferred to protoplanetary disks during star-formation, where they could be incorporated into nascent planetary systems. Understanding the fundamental processes in these ices can help us determine the fraction of prebiotic molecules that were delivered to our solar system from interstellar sources, and thereby establish the likelihood of their existence elsewhere in the universe.

Astrochemical models that are used to explain observed abundances of interstellar molecules rely on laboratory measurements to provide the necessary kinetic data. Such models typically implement large chemical kinetics networks and are often limited by the lack of quantitative laboratory data that exist.
The work in this thesis aims to provide constraints on various thermal and non-thermal processes at the surface, and within the bulk of astrochemically-relevant ice mantles. Thermal processes in ices include the diffusion, reaction and desorption of atoms and molecules that are thermalized to the dust temperature. Non-thermal processes involve the input of energy; typically from UV-photons, electrons or cosmic rays, that can induce diffusion, reactions or desorption.

In Chapter 3, we provide kinetic data for the formation of CO2 via the Eley-Rideal reaction between CO(g) and photoproduced OH radicals. This process provides a new route to the formation of CO2 in warm ice mantles that could be relevant in photon-dominated astrophysical environments. Chapter 4 presents a potential observational constraint on ice composition using the sensitivity of the CO2 longitudinal optical (LO) phonon modes to the ice mixing environment. Laboratory spectral measurements like this are especially critical with the upcoming launch of the James Webb Space Telescope (JWST), which will improve our understanding of the composition of icy dust grains and their evolution during star formation.

We also highlight the use of CO2 LO phonons in the laboratory by utilizing their sensitivity to the ice composition to follow CO diffusion into CO2 ices at low temperature. Coupling the extracted diffusion barrier with measurements of CO desorption from CO2 ices, we provide the diffusion-desorption barrier ratio for direct input into astrochemical models. These experiments show that the diffusion and desorption behaviour depends on the ice porosity, emphasizing the importance of understanding the morphology of ices in both the laboratory and in astrophysical environments.

Moving towards molecular complexity from the simple species studied in chapters 3-5, we conduct experiments to constrain photodestruction kinetics in simple ices and ice mixtures (chapter 6). UV-processing of ices is thought to be a dominant route to complex organic molecules; however, quantitative data of photo-induced processes in the solid phase are lacking. We report cross sections and quantum efficiencies for the ice photodestruction that can be input into astrochemical models to better constrain the formation of complex organic molecules.

Each of these chapters emphasize the importance of quantitative laboratory measurements of kinetic parameters associated with the mobility and reactivity of molecules in ices. Such measurements will become even more critical in the era of highly sensitive infrared telescopes like JWST.