Thermal desorption of astrophysically relevant molecules from forsterite(010)

TL;DR

This study investigates how small molecules like CO, CH4, O2, and CO2 desorb from forsterite(010) surfaces using temperature programmed desorption, revealing multilayer growth, desorption energies, and implications for astrochemical processes.

Contribution

It provides detailed desorption energy distributions for molecules on crystalline silicate surfaces, highlighting the impact of surface structure on molecule retention in space.

Findings

Multilayer growth occurs before monolayer saturation for CO, CH4, and O2.

Desorption energies are higher on crystalline forsterite than on other substrates.

Molecules can remain bound longer on crystalline silicate surfaces, affecting astrochemical models.

Abstract

We present temperature programmed desorption (TPD) measurements of CO, CH$_4$, O$_2$ and CO$_2$ from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of CO, CH$_4$ and O$_2$, multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On the other hand a single peak for CO$_2$ is observed which shifts from high temperature at low coverage to low temperature at high coverages, sharpening upon multilayer formation. The leading edges are aligned for all the molecules in the multilayer coverage regime indicating zero order desorption. We have extracted multilayer desorption energies for these molecules using an Arrhenius analysis. For sub-monolayer coverages, we observe an extended desorption tail to higher temperature. Inversion analysis has been used to extract the coverage dependent desorption energies in the sub-monolayer coverage regime, from which we obtain the desorption energy distribution. We found that owing to the presence of multiple adsorption energy sites on the crystalline surface the typical desorption energies of these small molecules are significantly larger than obtained in previous measurements for several other substrates. Therefore molecules bound to crystalline silicate surfaces may remain locked in the solid state for a longer period of time before desorption into the gas phase.