Polycyclic Aromatic Hydrocarbons (PAHs) are known to dominate the infrared spectra of a wide range of objects in the Universe. The earlier space missions, the Infrared Space Observatory and the Spitzer Space Observatory have revealed the spectral richness of the mid-infrared spectrum and have been critical in revealing the ubiquitous nature of PAHs in the Universe. Now, the James Webb Space Telescope (JWST), with its unprecedented spectral resolution and wavelength coverage for PAHs, is set to revolutionize the field of PAH astrochemistry. In this talk, I will give an overview of the richness of PAH emission as seen in high-resolution spectra of the Orion Bar obtained through the ERS program PDRS4All ID:1288. This new JWST data emphasizes the importance of sophisticated theoretical models in fully understanding the molecular physics underlying PAH emission. One of the limitations of developing a theoretical model of PAHs is the lack of experimental data on PAH characteristics. In this talk, I will also discuss the relevant molecular data required to develop the photochemical evolution model of PAHs.

Emission from polycyclic aromatic hydrocarbons (PAHs) is believed to dominate the infrared (IR) spectra of a wide variety of astronomical objects and environments. Quantum chemically computed PAH spectra, combined with experimental studies, are indispencible to analyze and interpret astronomical observations. To provide a foundation for the analysis of the high-fidelity JWST data, new computational tools have been developed at NASA Ames to produce fully anharmonic IR absorption and cascade emission spectra of various PAHs, including deuterated species. The substituted, aromatic molecule, phenylacetylene, is used as a test case for validation of the newly developed theoretical methods via comparison with IR absorption and emission experiments. This work sets the stage for future implementation of the code as a tool for populating the NASA Ames PAH IR Spectroscopic Database (PAHdb) with anharmonic spectra of vast numbers of PAHs for use in the analysis of astronomical PAH data.

Polycyclic aromatic hydrocarbons (PAHs) are abundant organic molecules detected in several objects in the universe, such as molecular clouds in the interstellar medium (ISM) L.J. Allamandola et al., The Astrophysical Journal 290, L25-L28 (1985). Their structure can be modified through plasma-driven processes occurring in the ISM. The present study focuses on the dissociation of naphthalene (C₁₀H₈) in a radiofrequency (RF) plasma, probed using cavity ringdown spectroscopy (CRDS) in the near-infrared. Namely, the low-power RF plasma source, called Platypus, is adapted from a small plasma thruster (“Pocket Rocket”) designed by the Space Plasma Power and Propulsion laboratory of the ANU C. Charles and R. W. Boswell. Plasma Sources Science and Technology, 21.2, 022002 (2012). A stable supersonic jet plasma is generated by expanding a mixture of argon and dissociated C₁₀H₈ into a vacuum chamber through a 20 mm long, 4 mm wide slit nozzle E. Dudás, Ph.D Thesis, 149-162 (2021). The jet-cooled fragmented C₁₀H₈ is finally probed with the ultra-sensitive CRDS technique. We recorded a spectrum from 5950 to 6120 cm⁻¹ composed of several hundred transitions originating from many different molecules, radicals, and probably ions M. Alliati et al., The Journal of Physical Chemistry A 123.10, 2107-2113 (2019).

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L.J. Allamandola et al., The Astrophysical Journal 290, L25-L28 (1985).. C. Charles and R. W. Boswell. Plasma Sources Science and Technology, 21.2, 022002 (2012).. E. Dudás, Ph.D Thesis, 149-162 (2021).. M. Alliati et al., The Journal of Physical Chemistry A 123.10, 2107-2113 (2019)..

Efforts over 40 years still leave the source of astronomical infrared emission bands largely unidentified. We report the first laboratory infrared (6–25 μm) spectra of gas-phase fullerene-metal complexes, [-Metal]⁺ (Metal = Fe, V), and show with density functional theory calculations that complexes of with cosmically abundant metals, including Li, Na, K, Mg, Ca, Al, V, Fe, all have similar spectral patterns. Comparison with observational infrared spectra from several fullerene-rich planetary nebulae demonstrates a strong positive linear cross-correlation. The infrared features of [-Metal]⁺ coincide with four bands attributed earlier to neutral bands, and in addition also with several to date unexplained bands.

Fullerenes (C₆₀, C₇₀ and C₆₀⁺) have been found to be the largest molecular species identified to date in the interstellar medium (ISM), and it has already been suggested that protonated C₆₀ and its complex analogues are among the most abundant C₆₀ analogues present in space. In a recent paper Nat. Astron. 4, 240–245 (2020) we presented the first experimental IR spectrum of C₆₀H⁺, although we were at the time unable to record the spectrum in the 3 μm range. In this work, the vibrational spectra of gaseous protonated and deuterated C₆₀ are recorded via infrared multiple-photon dissociation (IRMPD) for the first time in the CH and CD stretching region using the free electron laser FELIX interfaced with a quadrupole ion trap. In addition to the CH stretch band, the spectrum of C₆₀H⁺ shows in the 1600-3000 cm⁻¹ range the presence of other bands, which could be tentatively assigned as combination bands and overtones. The bands observed in this region are obviously weak but well-resolved, entailing that they may be excellent diagnostic features for protonated C₆₀. In fact, the single CH stretching band of C₆₀H⁺ falls at a frequency that is significantly lower than the CH stretching mode of aliphatic C-H bonds. Comparison of the IR spectra of several ionized fullerene analogues to IR emission spectra from planetary nebulae suggests that these species may exist in significant amounts in the ISM and be responsible for the unidentified interstellar features.

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Nat. Astron. 4, 240–245 (2020),

Polycyclic aromatic hydrocarbons (PAHs) are thought to be a major constituent of astrophysical environments, being the carriers of the ubiquitous aromatic infrared bands (AIBs) observed in the spectra of galactic and extra-galactic sources that are irradiated by ultraviolet (UV) photons. Small (2-cycles) PAHs were unambiguously detected in the TMC-1 dark cloud, showing that PAH growth pathways exist even at low temperatures. The processing of PAHs by UV photons also leads to their fragmentation, which has been recognized in recent years as an alternative route to the generally accepted bottom-up chemical pathways for the formation of complex hydrocarbons in UV-rich interstellar regions. Here we consider the C₁₂H₈⁺ ion that is formed in our experiments from the dissociative ionization of anthracene and phenanthrene (C₁₄H₁₀) molecules. By employing infrared pre-dissociation (IRPD) spectroscopy in a cryogenic ion trap instrument coupled to the free-electron lasers at the FELIX Laboratory, we have recorded broadband and narrow line-width gas-phase IR spectra of the parent (C₁₄H₁₀⁺) and fragment (C₁₂H₈⁺) ions and also reference spectra of three low energy isomers of the latter. By comparing the experimental spectra to those obtained from quantum chemical calculations we have identified the dominant structure of the fragment ion to be the acenaphthylene cation for both isomeric precursorsBanhatti et al., Phys. Chem. Chem. Phys. 24 (2022) 27343 Ab initio molecular dynamics simulations are presented to elucidate the fragmentation process. This result reinforces the dominant role of species containing a pentagonal ring in the photochemistry of small PAHs.

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Banhatti et al., Phys. Chem. Chem. Phys. 24 (2022) 27343.

l0pt Figure The search for the chemical structures that account for diffuse interstellar bands (DIBs) remains the main challenge in astrochemistry. It relies on experimental verification of structures obtained from stable isomers in laboratory conditions. The actual interstellar medium (ISM), however, is exposed to much harsher conditions. Herein we demonstrate the isomerization of small, doubly charged polyaromatic hydrocarbon phenanthrene obtained by electron ionization at relatively high electron energies. We performed electronic and vibrational tagging photodissociation spectroscopy measurements. Our results show that phenanthrene dication has a minor C₁₄H₁₀²⁺ isomer, originating from the isomerization of phenanthrene dication during the harsh ionization process. Hole-burning spectroscopy experiments provided the IR spectrum of an isomer which could be assigned based on DFT calculations as a fully conjugated system representing the global minimum on the C₁₄H₁₀²⁺ potential energy surface. The found isomer has no neutral closed-shell form and can only exist as a radical or as an ion. Finally, the IR spectrum of this isomer reveals features characteristic for the ISM, suggesting that it can potentially be a candidate for the DIB search. These results open new directions in searching for potential DIB candidates.

Cationic benzylium and tropylium are two competitive isomers formed by dissociative ionization of methylbenzene (toluene). Infrared predissociation (IRPD) spectroscopy of ions tagged with Ne has been established as a powerful tool in disentangling these cationic speciesJusko et al., ChemPhysChem 19 (2018) 3182 Methylated polycyclic aromatic hydrocarbons (PAHs) are expected to be abundant in space and their dissociative ionization could lead to the formation of both the benzylium- (XCH₂⁺) and tropylium-like (XC₇⁺) cations, which are expected to be the two lowest-energy isomers and whose isomerization process might impact their chemical evolution in photodissociation regions (PDRs). Here, we consider three methyl-PAHs as precursors, namely 1-methylpyrene, 2-methylnaphthalene, and 2-methylanthracene. Their cationic –H fragments, C₁₇H₁₁⁺, C₁₁H₉⁺, and C₁₅H₁₁⁺, were probed at the Free Electron Laser for Infrared eXperiments (FELIX) Laboratory using IRPD spectroscopy at the FELion cryogenic ion trap beamline. Their strongest vibrational band is located at about 6.2 μm revealing the predominance of the XCH₂⁺ isomersWenzel et al., J. Mol. Spectros. 385 (2022) 111620 Isomer abundance measurements and spectral comparison to computed anharmonic IR spectra show that only this isomer is present for C₁₇H₁₁⁺, whereas at least two isomers are present with a large abundance for the acene-derived species. Clear spectral evidence for the XC₇⁺ isomer is found in the C₁₁H₉⁺ case consistent with a relative abundance of 30 %. These results demonstrate the important role of steric hindrance in the formation of XC₇⁺ and reveal the potential of XCH₂⁺ to account for the aromatic IR emission band (AIB) observed at 6.2 μm in astrophysical environments, e.g. the planetary nebula NGC 7027 and the PDR at the Orion Bar.

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Jusko et al., ChemPhysChem 19 (2018) 3182. Wenzel et al., J. Mol. Spectros. 385 (2022) 111620.