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CO Diffusion on Interstellar Amorphous Solid Water: A Computational Study

Francesco Benedetti, Mauro Satta, Tommaso Grassi, Stefan Vogt-Geisse, Stefano Bovino

TL;DR

The study addresses uncertain diffusion parameters for interstellar ices by performing quantum-chemical calculations of CO diffusion on ASW clusters using DFT and NEB to map diffusion pathways, followed by harmonic transition state theory–based rate calculations across 10–100 K. It reveals a broad distribution of diffusion barriers with an average near 0.47 kcal/mol and a mean diffusion-to-binding energy ratio of about 0.27, not universally tied to Eb, indicating higher mobility than commonly assumed. The results align with low-end experimental data and imply that CO mobility on ASW could accelerate surface synthesis of complex organics and influence desorption dynamics and snowlines in disks. The work provides site-specific kinetic parameter distributions for astrochemical models and argues for replacing fixed alpha approximations with distribution-based approaches, with future extensions to other molecular species for broader implications in star- and planet-forming regions.

Abstract

Surface chemistry on interstellar dust grains is recognized as a central component in astrochemical models, representing a plausible formation route for many of the observed complex molecular species. However, key parameters governing interstellar surface chemistry, such as diffusion energy barriers, remain poorly constrained. In particular, surface diffusion constitutes a fundamental step for the synthesis of complex organic molecules and plays a crucial role in understanding the desorption process. In this paper, the diffusion dynamics of carbon monoxide (CO) on amorphous solid water (ASW) surfaces, representative of interstellar ices, is modeled with quantum-chemical methods. Employing a representative ensemble of water clusters, each made by 22 molecules, diffusion energy barriers between the binding sites are computed using Density Functional Theory. Diffusion rate coefficients are then determined by applying the harmonic approximation of Transition State Theory. The results, in agreement with experimental studies, revealed a wide distribution of diffusion energies. This reflects the intrinsic topological heterogeneity of ASW surfaces, and highlights how surface mobility significantly influences CO's desorption dynamics and, as a consequence, surface-mediated reactivity in interstellar environments. We show that key parameters commonly employed in astrochemical models, like the ratio between binding and diffusion energy, should be carefully revised.

CO Diffusion on Interstellar Amorphous Solid Water: A Computational Study

TL;DR

The study addresses uncertain diffusion parameters for interstellar ices by performing quantum-chemical calculations of CO diffusion on ASW clusters using DFT and NEB to map diffusion pathways, followed by harmonic transition state theory–based rate calculations across 10–100 K. It reveals a broad distribution of diffusion barriers with an average near 0.47 kcal/mol and a mean diffusion-to-binding energy ratio of about 0.27, not universally tied to Eb, indicating higher mobility than commonly assumed. The results align with low-end experimental data and imply that CO mobility on ASW could accelerate surface synthesis of complex organics and influence desorption dynamics and snowlines in disks. The work provides site-specific kinetic parameter distributions for astrochemical models and argues for replacing fixed alpha approximations with distribution-based approaches, with future extensions to other molecular species for broader implications in star- and planet-forming regions.

Abstract

Surface chemistry on interstellar dust grains is recognized as a central component in astrochemical models, representing a plausible formation route for many of the observed complex molecular species. However, key parameters governing interstellar surface chemistry, such as diffusion energy barriers, remain poorly constrained. In particular, surface diffusion constitutes a fundamental step for the synthesis of complex organic molecules and plays a crucial role in understanding the desorption process. In this paper, the diffusion dynamics of carbon monoxide (CO) on amorphous solid water (ASW) surfaces, representative of interstellar ices, is modeled with quantum-chemical methods. Employing a representative ensemble of water clusters, each made by 22 molecules, diffusion energy barriers between the binding sites are computed using Density Functional Theory. Diffusion rate coefficients are then determined by applying the harmonic approximation of Transition State Theory. The results, in agreement with experimental studies, revealed a wide distribution of diffusion energies. This reflects the intrinsic topological heterogeneity of ASW surfaces, and highlights how surface mobility significantly influences CO's desorption dynamics and, as a consequence, surface-mediated reactivity in interstellar environments. We show that key parameters commonly employed in astrochemical models, like the ratio between binding and diffusion energy, should be carefully revised.
Paper Structure (15 sections, 13 equations, 7 figures, 2 tables)

This paper contains 15 sections, 13 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology and computational details
  3. Geometrical optimizations and vibrational analysis
  4. Diffusion pathway analysis
  5. Diffusion and evaporation rate coefficients
  6. Results and discussion
  7. Selection of binding sites
  8. Diffusion rates calculation
  9. Comparison with desorption rates
  10. Astrophysical implications
  11. Conclusions
  12. Calculation of partition functions
  13. Rate temperature dependence
  14. Gaussian fit accuracy
  15. Astrophysical framework

Figures (7)

  • Figure 1: Energy profiles computed with the NEB method for 4 representative diffusion events: (\ref{['subfig:mep1']}), (\ref{['subfig:mep2']}), (\ref{['subfig:mep3']}) and (\ref{['subfig:mep4']}). In each plot, the red dots represent the energies of the 12 images (including the initial and final states), while the blue line indicates the MEP interpolation. Each panel uses an independent energy scale for better visualization.
  • Figure 2: Optimized structures (computed at M062X-D3/def2-TZVP level) of the initial and final binding sites and the corresponding transition state for the 4 representative diffusion events shown in Fig. \ref{['fig:meps']}: (\ref{['subfig:12_31']}), (\ref{['subfig:18_06']}), (\ref{['subfig:31_75']}) and (\ref{['subfig:13_09']}). Each subfigure shows the initial minimum (left), the transition state (center), and the final minimum (right).
  • Figure 3: Box plot comparison between the diffusion energy barriers for CO on ASW presented in this work and the values reported in previous investigations. The gray shaded area indicates the range of values obtained for $E_\mathrm d$ in this study.
  • Figure 4: (\ref{['subfig:ed_vs_eb']}) Distribution of the diffusion energy ($E_{\mathrm{d}}$) as a function of the binding energy ($E_{\mathrm{b}}$) for the selected pairs of adsorption sites. (\ref{['subfig:alpha_plot']}) Calculated values of $\alpha$ for the selected pairs of adsorption sites. Along with the obtained mean value, the two lines corresponding to the commonly assumed limiting values of $\alpha$ (0.3 and 0.7) are also plotted, and it is evident how the data is not strictly confined between the two limits. See further details in the main text.
  • Figure 5: Scatter plot between diffusion ($k_\mathrm d$) and desorption ($k_\mathrm e$) rate coefficients for the different pairs of binding sites at 10 K (\ref{['subfig:kd_vs_ke_10K']}) and 100 K (\ref{['subfig:kd_vs_ke_100K']}). For each plot, the KDE performed on the point distribution is shown as shaded contours. The dashed box in the left-side panel spans the region covered by the distribution at 100 K, highlighting the changes induced by the temperature variation.
  • ...and 2 more figures