The mobility of atoms and molecular radicals at ice-covered dust particles controls the surprisingly rich chemistry of circumstellar and interstellar environments, where a large number of different organic molecules have been observed, despite extremely low temperatures, pressures, and fluxes of radiation. Both thermal and non-thermal processes, for instance caused by UV radiation, have been inferred to play important roles. A growing number of experimental studies support previously suggested mechanisms and add to the understanding of possible astrochemical processes. Simulations, of both experiments and astrophysical environments, aid in interpreting experiments and suggesting important mechanisms. Still, the exact mechanisms behind the binding and mobility of species in interstellar ice are far from fully understood.We have performed atomic-scale calculations on mobility at water ice surfaces of varying morphology. Calculations of the interactions of H atoms with ice surfaces show that the experimentally observed slower diffusion at amorphous ice is due to considerably higher diffusion barriers than at crystalline ice. These results are in excellent agreement with recent experiments. It was also found that quantum tunneling is important for H atom mobility below 10 K.The binding energies and diffusion barriers of OH radicals at crystalline ice have been studied using the ONIOM(QM:AMOEBA) approach. Results indicate that OH diffusion over crystalline ice, contrary to the case of H atoms, might be slower at crystalline ice than at amorphous ice, due to a higher surface density of stronger binding sites at crystalline ice.We have also performed molecular dynamics simulations of the photoexcitation of vapor-deposited water at a range of surface temperatures. These results support that the experimentally observed desorption of H atoms following UV excitation is best explained by release of H atoms from photodissociation of water molecules in the deeper layers of the ice followed by efficient transport to the surface.

Atomic-scale simulations of atomic and molecular mobility in models of interstellar ice

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.