jz8b00328_si_001.pdf (2.85 MB)
Molecular Oxygen Formation in Interstellar Ices Does Not Require Tunneling
journal contribution
posted on 2018-03-26, 00:00 authored by Marco Pezzella, Oliver T. Unke, Markus MeuwlyThe formation of
molecular oxygen in and on amorphous ice in the
interstellar medium requires oxygen diffusion to take place. Recent
experiments suggest that this process involves quantum tunneling of
the oxygen atoms at sufficiently low temperatures. Fitting experimental
diffusion rates between 6 and 25 K to an expression that accounts
for the roughness of the surface yields excellent agreement. The molecular
dynamics of adsorbed oxygen is characterized by rapid intrasite dynamics,
followed by intersite transitions over distances of ∼10 Å.
Explicit simulations using a realistic free-energy surface for oxygen
diffusion on amorphous ice down to 10 K show that quantum tunneling
is not required for mobility of adsorbed oxygen. This is confirmed
by comparing quantum and classical simulations using the same free-energy
surface. The ratio of diffusional and desorption energy Edif/Edes = 275/1082 ≈
0.3 is at the lower end of typically used values but is still consistent
with the assumptions made in models for interstellar chemistry.