Binding Energy of Molecules on Water Ice: Laboratory Measurements and Modeling

TL;DR

This study provides precise measurements of molecular binding energies on water ice surfaces, revealing higher values at low coverage than previously used, impacting astrochemical models of molecule desorption.

Contribution

It offers new, accurate binding energy data for key molecules on different water ice types, improving astrochemical modeling accuracy.

Findings

Higher binding energies at low coverage than previous estimates.

N$_2$ and CO form clusters on water ice surfaces.

Molecules remain longer on grains due to higher desorption energies.

Abstract

We measured the binding energy of N$_2$, CO, O$_2$, CH$_4$, and CO$_2$ on non-porous (compact) amorphous solid water (np-ASW), of N$_2$ and CO on porous amorphous solid water (p-ASW), and of NH$_3$ on crystalline water ice. We were able to measure binding energies down to a fraction of 1\% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO$_2$ forms clusters on np-ASW surface even at very low coverages. The binding energies of N$_2$, CO, O$_2$, and CH$_4$ decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot core models. We found that owing to the higher value of desorption energy in the sub-monlayer regime a fraction of all these ices stays much longer and up to higher temperature on the grain surface compared to the single value energies currently used in the astrochemical models.