A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations

TL;DR

This study provides the first direct measurements of atomic oxygen desorption energy from dust grain analogs, revealing values about twice those previously assumed, which significantly impacts models of interstellar chemistry.

Contribution

It introduces the first experimental determination of O desorption energy from dust analogs and explores its effects on molecular cloud chemistry.

Findings

Desorption energies are approximately 1660 K for water ice and 1850 K for silicate surfaces.

Higher binding energies lead to increased water formation on grains at lower extinction levels.

The new values alter predictions of molecular abundances like H2O and O2 in interstellar clouds.

Abstract

The energy to desorb atomic oxygen from an interstellar dust grain surface, $E_{\rm des}$, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of $E_{\rm des}$ for atomic oxygen from dust grain analogs. The values of $E_{\rm des}$ are $1660\pm 60$~K and $1850\pm 90$~K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For $n=10^4$ cm$^{-3}$ and $G_0$=10$^2$ ($G_0$=1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H$_2$O can form on grains at lower visual extinction $A_{\rm V}$, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H$_2$O on grains, which are subsequently desorbed by FUV radiation, with consequences for gas-phase chemistry. For higher values of $n$ and $G_0$, the higher binding energy can lead to a large increase in the column of H$_2$O but a decrease in the column of O$_2$.