CO Diffusion on Interstellar Amorphous Solid Water: A Computational Study

Surface chemistry on interstellar dust grains is recognized
as
a central component in astrochemical models, representing a plausible
formation route for many of the observed complex molecular species.
However, key parameters governing interstellar surface chemistry,
such as diffusion energy barriers, remain poorly constrained. In particular,
surface diffusion constitutes a fundamental step in the synthesis
of complex organic molecules and plays a crucial role in understanding
the desorption process. In this paper, the diffusion dynamics of carbon
monoxide (CO) on amorphous solid water (ASW) surfaces, representative
of interstellar ices, is modeled with quantum-chemical methods. Employing
a representative ensemble of water clusters, each made of 22 molecules,
diffusion energy barriers between the binding sites are computed using
density functional theory. Diffusion rate coefficients are then determined
by applying the harmonic approximation of the transition state theory.
The results, in agreement with experimental studies, revealed a wide
distribution of diffusion energies. This reflects the intrinsic topological
heterogeneity of ASW surfaces and highlights how surface mobility
significantly influences CO’s desorption dynamics and, as a
consequence, surface-mediated reactivity in interstellar environments.
We show that key parameters commonly employed in astrochemical models,
such as the ratio between binding and diffusion energy, should be
carefully revised.