CO diffusion into amorphous H2O ices
  The mobility of atoms, molecules and radicals in icy grain mantles regulate
ice restructuring, desorption, and chemistry in astrophysical environments.
Interstellar ices are dominated by H2O, and diffusion on external and internal
(pore) surfaces of H2O-rich ices is therefore a key process to constrain. This
study aims to quantify the diffusion kinetics and barrier of the abundant ice
constituent CO into H2O dominated ices at low temperatures (15-23 K), by
measuring the mixing rate of initially layered H2O(:CO2)/CO ices. The mixed
fraction of CO as a function of time is determined by monitoring the shape of
the infrared CO stretching band. Mixing is observed at all investigated
temperatures on minute time scales, and can be ascribed to CO diffusion in H2O
ice pores. The diffusion coefficient and final mixed fraction depend on ice
temperature, porosity, thickness and composition. The experiments are analyzed
by applying Fick's diffusion equation under the assumption that mixing is due
to CO diffusion into an immobile H2O ice. The extracted energy barrier for CO
diffusion into amorphous H2O ice is ~160 K. This is effectively a surface
diffusion barrier. The derived barrier is low compared to current surface
diffusion barriers in use in astrochemical models. Its adoption may
significantly change the expected timescales for different ice processes in
interstellar environments.
