Influence of surface coverage on the chemical desorption process

TL;DR

This study investigates how surface coverage influences chemical desorption efficiency of molecules formed on dust grains, revealing a strong dependence on coverage and reaction pathway, with implications for astrochemical models.

Contribution

It introduces a new method to analyze chemical desorption ratios and demonstrates the significant impact of surface coverage on desorption efficiency for specific reactions.

Findings

Chemical desorption efficiency varies by reaction pathway by one order of magnitude.

Desorption efficiency for O+O reaction is about 80% at zero coverage, decreasing to near zero at full monolayer.

Surface coverage significantly affects chemical desorption, as shown by varying N₂ pre-adsorption.

Abstract

In cold astrophysical environments, some molecules are observed in the gas phase whereas they should have been depleted, frozen on dust grains. In order to solve this problem, astrochemists have proposed that a fraction of molecules synthesized on the surface of dust grains could desorb just after their formation. Recently the chemical desorption process has been demonstrated experimentally, but the key parameters at play have not yet been fully understood. In this article we propose a new procedure to analyze the ratio of di-oxygen and ozone synthesized after O atoms adsorption on oxidized graphite. We demonstrate that the chemical desorption efficiency of the two reaction paths (O+O and O+O$_2$) is different by one order of magnitude. We show the importance of the surface coverage: for the O+O reaction, the chemical desorption efficiency is close to 80 $\%$ at zero coverage and tends to zero at one monolayer coverage. The coverage dependence of O+O chemical desorption is proved by varying the amount of pre-adsorbed N$_2$ on the substrate from 0 to 1.5 ML. Finally, we discuss the relevance of the different physical parameters that could play a role in the chemical desorption process: binding energy, enthalpy of formation, and energy transfer from the new molecule to the surface or to other adsorbates.