Application of a diffusion–desorption rate equation model in astrochemistry

Abstract

Desorption and diffusion are two of the most important processes on interstellar grain surfaces; knowledge of them is critical for the understanding of chemical reaction networks in the interstellar medium (ISM). However, a lack of information on desorption and diffusion is preventing further progress in astrochemistry. To obtain desorption energy distributions of molecules from the surfaces of ISM-related materials, one usually carries out adsorption-desorption temperature programmed desorption (TPD) experiments, and uses rate equation models to extract desorption energy distributions. However, the often-used rate equation models fail to adequately take into account diffusion processes and thus are only valid in situations where adsorption is strongly localized. As adsorption-desorption experiments show that adsorbate molecules tend to occupy deep adsorption sites before occupying shallow ones, a diffusion process must be involved. Thus, it is necessary to include a diffusion term in the model that takes into account the morphology of the surface as obtained from analyses of TPD experiments. We take the experimental data of CO desorption from the MgO(100) surface and of D2 desorption from amorphous solid water ice as examples to show how a diffusion-desorption rate equation model explains the redistribution of adsorbate molecules among different adsorption sites. We extract distributions of desorption energies and diffusion energy barriers from TPD profiles. These examples are contrasted with a system where adsorption is strongly localized--HD from an amorphous silicate surface. Suggestions for experimental investigations are provided.

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

“…The TPD traces in Figure 1 share a common trailing edge, which indicates that the diffusion rate of CO is high and equilibrium diffusion state is reached during the desorption (He & Vidali 2014). Molecules tend to occupy the deep adsorption sites (with higher binding energy) before they fill shallow sites (with lower binding energy).…”

Sticking of Molecules on Nonporous Amorphous Water Ice

“…E ML and E LC are binding energies at monolayer coverage and low coverage, respectively. The E LC values for H 2 and D 2 are taken from Katz et al (1999) and He & Vidali (2014), respectively. The rest are taken from He et al (2016).…”

Surface Science Investigations of Icy Mantle Growth on Interstellar Dust Grains in Cooling Environments

“…31 Work by Dulieu, Lemaire, Kay, McCoustra, Vidali, and Zacharias illustrates this procedure. 23,[32][33][34][35][36] Table 1 summarises this state-of-the-art in TPD.…”