Session C26: The Chemical Physics of Molecules in Space I

Live

One of the most surprising findings of astrochemistry has been the variety and comparative complexity of the organic molecules detected in interstellar environments. The dominant theory regarding how these complex organic molecules (COM) came to be has been that they form from radicals trapped in dust-grain ice mantles, which begin to diffuse more efficiently as a collapsing cloud heats up. Over the last decade, though, the increasingly frequent detections of COMs in cold molecular clouds has led to questions regarding whether this thermal diffusion-dominated picture of their formation tells the whole story.

In the lab, COMs are readily produced within ices, at temperatures as low as 5 K, upon irradiation by, e.g., energetic ions, electrons, or UV photons - all of which undoubtedly bombard cosmic ices to varying degrees. However, incorporating the physical and chemical effects that lead to COM formation under these conditions has been challenging, due in part to the variety and complexity of the underlying processes. Over the last several years, we have endeavored to rectify this deficiency in astrochemical modeling by deriving new methods for simulating (a) cosmic ray-driven radiation chemistry, (b) the reactions of reactive species within ices, and most recently, (c) solid-phase UV photochemistry. These three areas will be the focus of this talk, as well as the revised picture of the evolution of COMs in the ISM that is suggested by our preliminary results.   

Live

Formation and destruction of simple and complex organic molecules in the solid phase in space is strictly linked to the physicochemical conditions at play. In the dense cold interstellar medium, dust particles are covered by polar and apolar ice layers that act as catalytic surfaces toward a higher molecular complexity. External triggers such as impinging atoms, photons, electrons and ions inject energy in a processed layer causing physical and chemical changes within the ice. To fully understand processes such as molecular diffusion, reaction, and desorption, it is therefore fundamental to investigate energy transfers mechanisms under interstellar conditions. In this talk, I will present recent experimental results on hydrogen bonding irreversible changes in water-rich ices induced by the intense, nearly monochromatic mid-IR and THz FEL radiation of the FELIX 1 and 2 beamlines. Experiments are carried-out by means of the ultrahigh vacuum (UHV) Laboratory Ice Surface Astrophysics (LISA) end-station designed and operated by my group at FELIX Laboratory. Current analytical tools of LISA are Fourier Transform Infrared spectroscopy and quadrupole mass spectrometry. Experiments are complemented by Molecular Dynamics simulations to constrain the effect at the molecular level.   

Live

Modeling the chemistry happening on top of interstellar dust grains requires information on several properties (sticking coefficients, binding energies, etc.) of adsorbates on surfaces. With the conditions of dense clouds, solid bodies are found as amorphous ices, where the quantities mentioned above are not univocally defined due to the distribution of binding sites on the surface. Therefore, these quantities' theoretical determination requires sampling of the expected outcomes on the surface's different sites. Here, we present our recent results on the study of these adsorption processes by combining sampling using ab-initio molecular dynamics simulations with the help of cheap electronic structure solvers [1]. We will focus our attention on using machine-learned potentials [2, 3] to affordably tackle these problems.

[1] Molpeceres, G., & Kästner, J. (2020). Phys. Chem. Chem. Phys., 22(14), 7552–7563.
[2] Zaverkin, V., & Kästner, J. (2020). J. Chem. Theory Comp., 16(8), 5410–5421.
[3] Molpeceres, G., Zaverkin, V., & Kästner, J. (2020). Mon. Not. Royal Astron. Soc., 499(1), 1373–1384.   

Live

Silicate nanoclusters are likely to be abundant in numerous astrophysical environments.The primary means to obtain information about astronomical silicate dust is through its infrared (IR) spectra, and comparison with experimentally prepared bulk silicates. However, silicate nanoclusters have not been experimentally produced yet, and their IR spectra should otherwise be determined to provide a better understanding of the extent of their astronomical importance.

Herein, we assess various computational methods for obtaining IR spectra of silicate dust grains from their atomistic structure and their atomic motions. First, we obtain the most stable silicate nanoclusters from global optimization searches. IR spectra of these nanoclusters are then obtained using density functional theory (DFT) based calculations within the harmonic oscillator approximation. To check if anharmonic effects play a significant role, we obtain IR spectra from finite temperature ab initio molecular dynamics (AIMD) simulations.

Finally, we also study the temperature effect on the broadening of the obtained IR spectra peaks in larger nanosilicate grains with a range of crystallinities. In this case less computationally costly classical MD simulations are necessary due to the large number of atoms involved.   

Live

In order to provide reliable and detailed laboratory data to interpret observations of spectral features of interstellar ices and of outer solar system frozen worlds, it is necessary to characterize the mid IR features of pure and mixed ices in realistic simulations of interstellar and planetary environments. We studied the characteristics and thermal evolution of thin films of pure CH₄ and CH₄ mixed with water and N₂. These are systems that have been observed in interstellar space (CH₄ and mixed with H₂O) and solar system objects such as Pluto and Triton (CH₄ mixed with N₂). We show how the mixing ratio, temperature, and the thickness affect the IR spectral features of the ice. The nuclear spin conversion in the ice was also studied using IR modes of methane. We found that the segregation and crystallization of CH₄:H₂O and CH₄:N₂ mixtures in response to heating can be clearly characterized by mid-IR features, thus providing a tool to help the interpretation of observations.   

Live

The reaction of CO₂ and NH₃ to form ammonium carbamate (CH₆N₂O₂) is fairly well known. Thermal processing of mixtures of CO₂, NH₃, and other species have shown that ammonium carbamate can form and trap some species where they can remain well above their sublimation temperatures. Infrared absorption spectroscopy and a closed-cycle helium cryostat are used to create and analyze the ice mixtures at various temperatures. A series of experiments investigate the capability of ammonium carbamate to trap various molecules while also determining whether or not ammonium carbamate will form with NH₃ mixed with other carbonaceous species.   

Live

About 60 complex molecules have been observed in space. It is generally accepted that these are formed on the surface of dust grains in cold, dark regions of space. Here the grain acts both as an accretion spot for reactants and an energy sink for exothermic reactions. This concept forms the basis for theoretical models that are used in the interpretation of observational data in order to extract molecular details.

The dynamics of surface-reaction complexes are hitherto not considered in the application of such models. Previously, we showed that limited movement of reactants can yield non-reactive complexes that decrease reaction efficiency. Although, this effect is small for single-carbon baring species, we expect it to become more significant when larger species are considered.

Here we have used small molecules as a proxy to study the dynamics of reactants on cold grain surfaces. Metadynamics simulations give us binding sites that facilitate reactions. Using these binding sites we have simulated the meeting of small molecules on a surface. Finally, molecular dynamics simulations provided limits on the rearrangement of reactants in feasible binding sites. Altogether, these simulation results provide a clearer image of how species can meet on cold grain surfaces prior to reaction.   

Not Participating

Benzonitrile (C₆H₅CN), an aromatic molecule of interest to astrochemists, is reported to be present in the interstellar medium (ISM) [1]. The energetic processing of aromatic molecules can synthesize large and complex aromatic molecules such as the Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are considered to be the carriers of unknown InfraRed (IR) bands. To-date a number of laboratory experiments have reported the formation of complex organics from energetic processing of aromatic molecules [2-3].
In the present investigation, we subjected benzonitrile ice made at 4 K to vacuum ultraviolet (9 eV) radiation. After irradiation the ice was warmed to room temperature, which left a brownish residue on the KBr substrate. The VUV spectrum of the residue is observed to have characteristic aromatic signature. The residue is then transferred to a quantifoil grid for High-Resolution Transmission Electron Microscope (HR- TEM) imaging. HR-TEM micrographs revealed the presence of graphene in the residue. This result suggests that nitrogen-doped graphene (N-graphene) can be synthesized in regions of the ISM where benzonitrile is present.
[1] McGuire et al. (2018) Science, 359 (6372), 202. [2] Callahan et al. (2013) Icarus, 226, 1201. [3] Rahul et al. (2020) Spectrochim. Acta A, 231, 117797.