Efficiency of the top-down polycyclic aromatic hydrocarbon-to-fullerene conversion in ultraviolet irradiated environments

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and fullerenes play a major role in the physics and chemistry of the interstellar medium. Based on a number of recent experimental and theoretical investigations we developed a model in which PAHs are subject to photo-dissociation (carbon and hydrogen loss) and hydrogenation. We take into account that dehydrogenated PAHs may fold into closed structures – fullerenes. Fullerenes, in their turn, can be also hydrogenated, becoming fulleranes, and photo-dissociated, losing carbon and hydrogen atoms. The carbon loss leads to shrinking of fullerene cages to smaller ones. We calculate the abundance of PAHs and fullerenes of different sizes and hydrogenation level depending on external conditions: the gas temperature, intensity of radiation field, number density of hydrogen atoms, carbon atoms, and electrons. We highlight the conditions, which are favourable for fullerene formation from PAHs, and we conclude that this mechanism works not only in H-poor environment but also at modest values of hydrogen density up to 104 cm−3. We found that fulleranes can be formed in the ISM, although the fraction of carbon atoms locked in them can be maximum around 10−9. We applied our model to two photo-dissociation regions, Orion Bar and NGC 7023. We compare our estimates of the fullerene abundance and synthetic band intensities in these objects with the observations and conclude that our model gives good results for the closest surroundings of ionising stars. We also demonstrate that additional fullerene formation channels should operate along with UV-induced formation to explain abundance of fullerenes far from UV sources.

A plausible molecular mechanism to explain near-infrared continuum emission: Recurrent fluorescence

“…The abundance of cationic carbon clusters are expected to be comparable to that of neutral carbon clusters. For instance, Iglesias-Groth (2019) deduced an ionisation fraction of 20% for buckminsterfullerene in the IC 348 star-forming region, and Murga et al (2022) found that this fraction is mostly less than 80% in the Orion Bar. However, owing to their rather large size and the spreading of the extra charge over the entire carbon mesh, we did not expect significant changes in the vibrational or electronic structures, at least from the statistical perspective that we adopted in this work.…”

Hydrogenated amorphous carbon grains as an alternative carrier of the 9–13 μm plateau feature in the fullerene planetary nebula Tc 1

“…However, the formation route of fullerenes in such H-rich objects, similar to the majority of PNe, is not well understood. Nowadays, the most suitable routes for the formation of fullerenes are 1 : i) the photochemical processing of hydrogenated amorphous carbon (HAC) grains or similar mixed aromatic or aliphatic hydrocarbons (i.e., same chemical composition but a different internal structure; hereafter HAClike; García-Hernández et al 2010) and ii) the photochemical processing of large polycyclic aromatic hydrocarbons (PAHs; Berné & Tielens 2012;Murga et al 2022).…”

In search of universalities in the dissociative photoionization of PANHs via isomerizations

“…11 These molecules are also thought to play a very important role in influencing the local chemistry and contributing to the inventory of organic molecules in different regions of the universe. For instance, it is hypothesized that PAHs are responsible for the formation of fullerenes [12][13][14] in space like conditions. Laboratory studies 15 have also indicated that, via hydrogenation and hydroxylation reactions, they can be transformed into more complex molecules, even at temperatures as low as 5 K, typical of dense molecular clouds.…”