Peculiarities in the infrared emission of polycyclic aromatic hydrocarbon-C₆₀ adducts

¹ Departamento de Física and IUdEA, Universidad de La Laguna (ULL), 38200 Tenerife, Spain
² De Vinci Higher Education, De Vinci Research Center, 92 916 Paris, France
³ Institut Supérieur des Arts Multimédia de la Manouba, Université de la Manouba, 2010 la Manouba, Tunisia
⁴ Faculté des Sciences de Tunis, Département de Physique, (LPMC), Université de Tunis El Manar, 2092 Tunis, Tunisia
⁵ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92195 Meudon, France
⁶ Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, 38205 La Laguna, Spain
⁷ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Spain
⁸ Institut de Quimica Computacional i Catalisi (IQCC), Departament de Quimica, Universitat de Girona, Girona 17003, Catalonia, Spain
⁹ CIGUS CITIC – Department of Nautical Sciences and Marine Engineering, University of A Coruña, Paseo de Ronda 51, 15011 A Coruña, Spain
¹⁰ Consejo Superior de Investigaciones Científicas (CSIC), Spain
¹¹ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), c. Martí i Franqués, 1, 08028 Barcelona, Spain
¹² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), c. Martí i Franqués, 1, 08028 Barcelona, Spain
¹³ Department of Physics, College of Khurma University, Taif University, PO Box 11099, Taif 21944, Saudi Arabia

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Received: 4 February 2025
Accepted: 25 October 2025

Abstract

The coexistence of polycyclic aromatic hydrocarbons (PAHs) and the C₆₀ fullerene in different astrophysical environments can give rise to the formation of new complex species denoted as PAH-C₆₀ adducts, which may contribute to the infrared (IR) emission observed. These PAH-C₆₀ adducts have previously been reported experimentally due to the high reactivity between PAHs and C₆₀. From an astrophysical point of view, however, they have not been considered in detail yet. Here, we performed a combined experimental and theoretical study in order to characterize the IR spectra of PAH-C₆₀ adducts, including multiple adducts. Using new advanced experimental techniques, we were able to synthesize some specific PAH-C₆₀ adduct isomers and measure their IR spectra. These experimental data were used to correct their harmonic scaled spectra, as obtained from quantum-chemistry calculations performed at the density functional theory (DFT) level under the B3LYP-GD3/6-31+G(d) approach. This way, we simulated the IR (~3–25 μm) spectra of multiple PAH-C₆₀ adducts, composed by a different number of PAH units: mostly one or two units. In addition, the chemical kinetics data available in the literature were used to tentatively estimate the possible order of magnitude of the abundances of these PAH-C₆₀ adducts using the available observational data. Essentially, our results revealed a possible strong modification of the IR spectra when astronomically estimated abundances are considered. Several spectral peculiarities are observed, such as a broad ~3.4–3.6 μm feature, and important modifications in the 6–10 and 12–16 μm spectral regions together with contributions to the C₆₀ features at 7.0 and 18.9 μm. Interestingly, these PAH-C₆₀ adducts lack aliphatic CH bonds, but they display IR features around 3.4 μm, challenging previous interpretations of this astronomical feature.