Co Diffusion on ASW Achieves Key Insights into 22-Molecule Ice Chemistry

Scientists are increasingly recognising the importance of surface chemistry on interstellar dust grains in forming complex molecules observed in space. Francesco Benedetti, Mauro Satta, and Tommaso Grassi, from Sapienza University of Rome and the Max-Planck-Institut f ür Extraterrestrische Physik, alongside Stefan Vogt-Geisse and Stefano Bovino et al., have now investigated a crucial, yet poorly understood, aspect of this chemistry , the diffusion of carbon monoxide (CO) on amorphous solid water (ASW). Their computational study, employing Density Functional Theory on representative water clusters, reveals a surprisingly wide distribution of diffusion energies reflecting the disordered nature of interstellar ices. This research is significant because it challenges commonly used parameters in astrochemical models and provides vital insight into how molecules move and react on these surfaces, ultimately influencing the formation of complex organic compounds in interstellar environments.

This research, employing quantum-chemical methods, addresses a long-standing challenge in astrochemical modelling, the poorly constrained parameters governing surface chemistry on interstellar dust grains. These findings directly impact the accuracy of astrochemical models used to predict molecular formation in dense molecular clouds, where temperatures below 20 Kelvin and densities around 104 cubic centimetres favour chemistry occurring on icy surfaces.

Experiments show that surface diffusion is a fundamental process for the synthesis of complex organic molecules and is vital for understanding how molecules escape from icy surfaces, a process known as desorption. Researchers computed diffusion energy barriers between binding sites on the amorphous solid water, utilising the harmonic approximation of Transition State Theory to determine diffusion rate coefficients. The results align with existing experimental studies, confirming the significant topological heterogeneity of ASW surfaces and highlighting how surface mobility influences CO desorption dynamics and subsequent surface-mediated reactivity. This work establishes that commonly used parameters in astrochemical models, specifically the ratio between binding and diffusion energy, require careful revision to accurately reflect the complexities of interstellar ice environments.
The study unveils a wide distribution of diffusion energies for CO on ASW, a finding that challenges the common assumption of a constant relationship between binding and diffusion energies. Traditionally, astrochemical models approximate the diffusion energy barrier as a fixed fraction of the adsorption energy, employing values between 0.3 and 0.7; however, this research suggests a more nuanced approach is necessary. By employing a representative ensemble of water clusters, the team was able to capture the intrinsic variability of ASW surfaces, demonstrating that the energy required for CO to diffuse varies significantly across different binding sites. This detailed modelling provides a more realistic representation of the conditions within dense interstellar clouds, where the formation of complex molecules is crucial for the development of planetary systems.

This breakthrough reveals the importance of accurately modelling surface diffusion in astrochemical simulations, as it directly impacts the predicted rates of chemical reactions and the abundance of complex organic molecules. The research highlights the limitations of current astrochemical models that rely on simplified assumptions about diffusion processes, particularly the constant ratio between binding and diffusion energy. Furthermore, the work opens new avenues for refining these models by incorporating the observed heterogeneity of ASW surfaces and the resulting distribution of diffusion energies. Recent observations from the James Webb Space Telescope have provided unprecedented detail of interstellar ices, and this computational study provides a crucial theoretical framework for interpreting these observations and furthering our understanding of the chemical processes occurring in the cosmos.

CO Diffusion on Water Ice via DFT

Scientists investigated interstellar surface chemistry, focusing on carbon monoxide (CO) diffusion on amorphous solid water (ASW) surfaces to better understand complex molecule formation. The study employed quantum-chemical methods to model CO diffusion dynamics, utilising a representative ensemble of 22-molecule water clusters to simulate interstellar ices. This approach enabled the calculation of how quickly CO molecules move across the ASW surface, a crucial factor in interstellar reactivity. The work revealed a wide distribution of diffusion energies, reflecting the inherent topological heterogeneity of ASW surfaces and highlighting the significant influence of surface mobility on CO desorption dynamics. The study pioneered a detailed examination of the relationship between binding and diffusion energies, challenging commonly used parameters in astrochemical models. Researchers demonstrated that the ratio between binding and diffusion energy, often treated as a constant, requires careful revision for accurate modelling. Subsequently, they determined diffusion rate coefficients using the harmonic approximation of Transition State Theory, yielding results that align with existing experimental studies. These calculations revealed a wide distribution of diffusion energies, reflecting the inherent topological heterogeneity of ASW surfaces and highlighting the significant influence of surface mobility on CO desorption dynamics and reactivity within interstellar environments.

Experiments revealed a diverse range of diffusion energies, demonstrating that the commonly used assumption of a constant ratio between binding and diffusion energy in astrochemical models requires careful revision. The study computed diffusion energy barriers between binding sites on the ASW surfaces, utilizing a representative ensemble of water clusters, each comprising 22 molecules. Data shows that the diffusion energy for CO varies considerably across the ASW surface, a consequence of its disordered structure. This heterogeneity directly impacts the mobility of CO molecules and, consequently, their ability to participate in surface-mediated reactions crucial for the formation of complex organic molecules.

Researchers measured diffusion rate coefficients by applying the harmonic approximation of Transition State Theory, building upon the computed energy barriers. Results demonstrate that the pre-exponential factor, often assumed constant in models, may also vary depending on the specific surface conditions and molecular species. The breakthrough delivers a more nuanced understanding of diffusion processes on interstellar ices, challenging the simplification of using a fixed value for the ratio between binding and diffusion energy. Tests prove that the current astrochemical models may underestimate the complexity of surface chemistry occurring on interstellar dust grains.

The team’s work highlights the importance of considering the intrinsic heterogeneity of ASW surfaces when modeling interstellar chemistry. Measurements confirm that the diffusion energy for CO is not a single value but rather a distribution, influenced by the local environment of each binding site. This finding has significant implications for understanding the formation of complex molecules in dense molecular clouds, as it affects the encounter probability of reactants and the overall reaction rates. Subsequent application of the harmonic approximation of Transition State Theory then determined diffusion rate coefficients, revealing a wide distribution of diffusion energies consistent with experimental observations. The findings demonstrate significant heterogeneity in diffusion energies, ranging from almost zero up to 1.24 kcal mol−1, with an average of 0.47 kcal mol−1, a value notably lower than typically assumed in astrochemical models.

This suggests CO mobility on ASW surfaces could be substantially higher than previously thought, potentially impacting the formation of complex organic molecules (COMs) and desorption processes at temperatures common in molecular clouds (10, 20 K). The authors acknowledge a limitation in not extending their modelling to include multi-binding and multi-diffusion effects within planetary disks, leaving this for future work. Further research could extend this approach to different molecules, particularly radical species, to clarify the role of surface mobility in chemical evolution within star- and planet-forming regions.