CO diffusion into amorphous H2O ices

TL;DR

This study quantifies the diffusion rate and energy barrier of CO molecules into amorphous H2O ices at low temperatures, revealing a lower barrier than previously assumed, which impacts astrochemical models.

Contribution

It provides the first experimental measurement of CO diffusion barrier into amorphous H2O ice, highlighting its significance for astrochemical modeling.

Findings

CO diffuses into H2O ice pores at 15-23 K within minutes.

The diffusion barrier for CO into amorphous H2O is approximately 160 K.

The low diffusion barrier suggests faster ice chemistry processes in space.

Abstract

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.