CO diffusion and desorption kinetics in CO$_2$ ices

TL;DR

This study measures CO diffusion and desorption in CO2 ices at low temperatures, revealing surface diffusion mechanisms and providing key kinetic parameters relevant for interstellar chemistry modeling.

Contribution

It presents the first detailed measurements of CO diffusion coefficients and barriers in CO2 ices, highlighting surface pore diffusion over bulk diffusion and quantifying desorption energies.

Findings

Diffusion barrier of 300 ± 40 K for CO in CO2 ice.

CO desorption energy ranges from 1240 ± 90 K to 1410 ± 70 K.

Diffusion-desorption barrier ratio is approximately 0.22.

Abstract

Diffusion of species in icy dust grain mantles is a fundamental process that shapes the chemistry of interstellar regions; yet measurements of diffusion in interstellar ice analogs are scarce. Here we present measurements of CO diffusion into CO$_2$ ice at low temperatures (T=11--23~K) using CO$_2$ longitudinal optical (LO) phonon modes to monitor the level of mixing of initially layered ices. We model the diffusion kinetics using Fick's second law and find the temperature dependent diffusion coefficients are well fit by an Arrhenius equation giving a diffusion barrier of 300 $\pm$ 40 K. The low barrier along with the diffusion kinetics through isotopically labeled layers suggest that CO diffuses through CO$_2$ along pore surfaces rather than through bulk diffusion. In complementary experiments, we measure the desorption energy of CO from CO$_2$ ices deposited at 11-50 K by temperature-programmed desorption (TPD) and find that the desorption barrier ranges from 1240 $\pm$ 90 K to 1410 $\pm$ 70 K depending on the CO$_2$ deposition temperature and resultant ice porosity. The measured CO-CO$_2$ desorption barriers demonstrate that CO binds equally well to CO$_2$ and H$_2$O ices when both are compact. The CO-CO$_2$ diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on the binding environment during diffusion. The diffusion-desorption ratio is consistent with the above hypothesis that the observed diffusion is a surface process and adds to previous experimental evidence on diffusion in water ice that suggests surface diffusion is important to the mobility of molecules within interstellar ices.