Application of a diffusion-desorption rate equation model in astrochemistry

Area of Science:

• Astrochemistry and Surface Science
• Interstellar Medium (ISM) Processes
• Computational Modeling of Molecular Interactions

Background:

• Desorption and diffusion on interstellar grain surfaces are crucial for understanding chemical reaction networks in the ISM.
• Current rate equation models often neglect diffusion, limiting their accuracy for non-localized adsorption scenarios.
• Experimental evidence suggests molecules preferentially occupy deeper adsorption sites, indicating the involvement of diffusion.

Purpose of the Study:

• To develop and validate a diffusion-desorption rate equation model for analyzing adsorption-desorption processes on interstellar grain surfaces.
• To accurately extract desorption energy distributions and diffusion energy barriers from experimental data.
• To improve the understanding of molecular behavior on relevant ISM materials like MgO and amorphous solid water ice.

Main Methods:

• Utilized temperature programmed desorption (TPD) experimental data for CO from MgO(100) and D2 from amorphous solid water ice.
• Developed a novel diffusion-desorption rate equation model incorporating surface morphology.
• Applied the model to explain adsorbate molecule redistribution and extract energy distributions and diffusion barriers.

Main Results:

• The diffusion-desorption model successfully explains the redistribution of molecules among different adsorption sites.
• Distributions of desorption energies and diffusion energy barriers were extracted from TPD profiles for CO/MgO and D2/ice.
• Contrasted findings with a localized adsorption system (HD/silicate) to highlight the model's applicability.

Conclusions:

• Incorporating diffusion into rate equation models is essential for accurate astrochemistry studies involving non-localized adsorption.
• The developed model provides a more realistic framework for interpreting TPD experiments and understanding interstellar surface chemistry.
• Further experimental investigations are suggested to refine our understanding of diffusion-limited desorption processes in the ISM.