Finite-temperature stability of hydrocarbons: Fullerenes vs flakes

Pérez‐Mellor, Ariel; Parneix, P.; Calvo, F.; Falvo, Cyril

doi:10.1063/5.0122561

Cited by 3 publications

(2 citation statements)

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“…Regarding pure carbon clusters, quantum chemical calculations show that when size increases, the cages population becomes energetically more favorable than the flakes population (Dubosq et al 2019). However, interestingly, a recent theoretical study by Perez-Mellor et al (2022) shows that both entropy and addition of a moderate amount of hydrogen favors planar structures analogous to the flakes populations studied in this work. Typically, regarding the hydrogenated isomers of C 60 , the flakes population was determined as the most favorable for C 60 H n with 12 ≤ n ≤ 60 stoichiometries for T between 0 and 3500 K. Therefore, we may reasonably assume that the IR features for larger clusters such as C 60 H n with 12 ≤ n ≤ 60 are similar to the ones obtained for C 24 H 6 .…”

Section: Astrophysical Implicationsmentioning

confidence: 53%

Spectroscopic investigation of interstellar hydrogenated carbon clusters

Dubosq

Pla

Dartois

et al. 2023

A&A

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Context. The assignment of the mid-infrared (mid-IR) emission features and plateaus observed in C-rich and H-rich regions of the interstellar medium (ISM) is still debated. Such mid-IR assignments must also be related to their contribution to the extinction curve in our galaxy and to the ultraviolet (UV) bump. Aims. The aim of this work is to investigate the influence of hydrogenation rate on the mid-IR spectra of populations of carbon clusters in order to constraint the nH /nC ratios in regions of the ISM where carbon is an important component. Their potential contribution to the extinction curve and in particular to the UV bump is also investigated. Methods. The absorption IR and optical spectra of tens of thousands of C24Hn (n=0,6,12,18,24) isomers classified into structural families -namely flakes, branched, pretzels, and cages-were computed using the density functional based tight binding electronic structure method and its time-dependent version, respectively. Final spectra were obtained by averaging the spectra of many individual isomers. Results. The shapes and the relative intensities of the bands centered at ∼ 3.25 and 3.40 µm and assigned to the C-H stretch of sp 2 and sp 3 carbon atoms, respectively, present a clear dependence on the nH /nC ratio. From a comparison with the astronomical spectrum from the Orion bar H2S1, the most interesting emitting candidates would pertain to the flakes population; this is the most energetically favorable family of clusters, possessing a high content of five and six carbon rings and being mostly planar, with no sp 1 carbon atoms and with a nH /nC ratio of lower than 0.5. The same conclusion is drawn when comparing the computed IR features in the [4-20 µm] region with the observed plateaus from some C-rich and H-rich planetary nebulae objects of the Small Magellanic Cloud. The contribution of the same family could be considered for the UV bump. When nH /nC increases, only a contribution to the high-energy part of the continuum due to σ → π excitations can reasonably be considered. Conclusions. These results bring some constraints on the structural features and on the nH /nC ratio of the hydrogenated carbon populations emitting in the mid-IR domain in interstellar objects such as protoplanetary and reflection nebulae. The flakes population, with a low nH /nC ratio, is an interesting candidate for the carbon population emitting in these regions, but not for that absorbing in the diffuse ISM. None of the populations studied in the present work can account for the UV bump, but they would contribute to a broad extinction rise in this domain. The computed features reported in this article could be used to interpret future astronomical data provided by the James Webb Space Telescope.

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Section: Astrophysical Implicationsmentioning

confidence: 53%

Spectroscopic investigation of interstellar hydrogenated carbon clusters

Dubosq

Pla

Dartois

et al. 2023

A&A

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“…The RF from large PAHs with low-lying electronic states could also contribute to the near-IR continuum emission, and its correlation to AIBs would be worth investigating further. Considering that PAHs are stabilised over cages due to a few extra hydrogen atoms (Perez-Mellor et al 2022;Lepeshkin et al 2022), applying the present statistical framework to those compounds would appear to be a natural extension of this work. In Dubosq et al (2020), the electronic oscillator strength of all S n → S 0 transitions were determined using the timedependent DFTB approach.…”

Section: Discussionmentioning

confidence: 99%

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

Lacinbala

Calvo

Dartois

et al. 2023

A&A

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Context. Very small grains and large hydrocarbon molecules are known to convert a fraction of the ultraviolet (UV) and visible stellar radiation to near- and mid-infrared (IR) photons via stochastic heating and subsequent radiative de-excitation. However, no convincing explanation for the near-IR continuum emission observed in some reflection nebulae and planetary nebulae has been provided so far. Aims. We aim to investigate the extent that recurrent fluorescence originating from stellar photon absorption by Cn (n = 24, 42, 60) carbon clusters can account for the IR emission detected in various interstellar environments. To this aim, we modelled the collective emission signature of a carbon cluster sample induced by irradiation from a 20 000 K blackbody source. From the obtained results, we set out to determine the fraction of interstellar carbon locked up in the emitting objects. Methods. The collective emission signature was computationally determined for different structural families encompassing cages, flakes, pretzels, and branched isomers by means of a kinetic Monte Carlo stochastic approach based on harmonic vibrational densities of states. The collective emission spectra result from the overall radiative cooling of a large population of neutral carbon clusters, during which recurrent fluorescence and vibrational emission compete with each other. Results. Our modelling shows that recurrent fluorescence from C60 cages and flakes (with little or no sp1 carbon atoms) and C42 cages are able to explain the near-IR continuum emission observed in several reflection nebulae and planetary nebulae. Assuming that the continuum emission observed towards NGC 7023 is due to recurrent fluorescence induced by UV or visible photon absorption in neutral cage carbon clusters containing about 30–60 atoms, the carriers contain about 0.1–1.5% of the interstellar carbon abundance.

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Investigation of fullerene cluster growth mechanisms by carbon atom addition using classical molecular dynamics

Allouch,

Mougenot,

Michau

et al. 2023

The Journal of Chemical Physics

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The mechanisms of carbon sticking reactions to C36 and C–C80 fullerenes were investigated with molecular dynamics simulations (MD) using the Second-generation Reactive Empirical Bond Order (SREBO) and Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potentials that were specifically optimized for carbon-carbon interactions. Results showed the existence of three possible sticking configurations where the projectile atom can stick either to one, two or three atoms of the target fullerene. They also showed that although the two potentials give similar magnitudes for the sticking cross-sections, they yield fairly different results as far as sticking mechanisms and configurations at thermal collision-energies, i.e., in the range 0.05–0.5 eV, are concerned. While AIREBO, that takes into account the long-range Lennard-Jones interaction, essentially results in a surface-sticking configuration with a single atom of the target fullerene, SREBO potential yields both surface- and two neighbors-sticking (2N-sticking) configurations. The fullerene structure is preserved in the last configuration while it can be recovered by a 2000 K annealing in the former configuration. Results obtained with SREBO eventually showed larger sticking probabilities for C36 as compared with C80. In spite of this, the sticking cross-sections obtained for C80 are similar to or even larger than those obtained for C36 due to the larger size of C80 that compensates for its smaller sticking probabilities.

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Finite-temperature stability of hydrocarbons: Fullerenes vs flakes

Cited by 3 publications

References 61 publications

Spectroscopic investigation of interstellar hydrogenated carbon clusters

Spectroscopic investigation of interstellar hydrogenated carbon clusters

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

Investigation of fullerene cluster growth mechanisms by carbon atom addition using classical molecular dynamics

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