Theoretical evaluation of PAH dication properties

Malloci, Giuliano; Joblin, C.; Mulas, G.

doi:10.1051/0004-6361:20066053

Cited by 74 publications

(75 citation statements)

References 89 publications

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“…6, the calculated cation IEs show the same propensity as the calculated neutral IEs for underestimating the experimental values and the same downward trend with increasing system size. In the work of Malloci and co-workers (Malloci et al 2007) Doubly ionized species are subject to dissociation by coulombic forces. As such, it is accepted that second ionization will lead to destruction of a molecule in the diffuse medium (Allain et al 1996).…”

Section: Resultsmentioning

confidence: 99%

On the electronic spectroscopy of closed-shell cations derived from resonance-stabilized radicals: Insights from theory and Franck-Condon analysis

Troy

Kable

Schmidt

et al. 2012

A&A

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Context. Recent attention has been directed on closed-shell aromatic cations as potential carriers of the diffuse interstellar bands. The spectra of mass-selected, matrix-isolated benzylium, and tropylium cations were recently reported. The visible spectrum of benzylium exhibits a large Franck-Condon (FC) envelope, inconsistent with diffuse interstellar band carriers. Aims. We perform a computational analysis of the experimentally studied benzylium spectrum before extending the methods to a range of larger, closed-shell aromatic cations to determine the potential for this class of systems as diffuse interstellar band carriers. Methods. Density functional theory (DFT), time-dependant ((TD)DFT), and multi-configurational self-consistent field second-order perturbation theory (MRPT2) methods in concert with multidimensional FC analysis is used to model the benzylium spectrum. These methods are extended to larger closed-shell aromatic hydrocarbon cations derived from resonance-stabilized radicals, which are predicted to show strong S 0 → S n transitions in the visible region. The ionization energies of a range of these systems are also calculated by DFT. Results. The simulated benzylium spectrum was found to yield excellent agreement with the experimental spectrum showing an extended progression in a low frequency (510 cm −1 ) ring distortion mode. The FC progression was found to be significantly quenched in the larger species: 1-indanylium, 1-naphthylmethylium, and fluorenium. Excitation and ionization energies of the closed-shell cations were found to be consistent with diffuse interstellar band carriers, with the former lying in the visible range and the latter straddling the Lyman limit in the 13−14 eV range. Conclusions. Large closed-shell polycyclic aromatic hydrocarbon cations remain viable candidate carriers of the diffuse interstellar bands.

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Section: Resultsmentioning

confidence: 99%

On the electronic spectroscopy of closed-shell cations derived from resonance-stabilized radicals: Insights from theory and Franck-Condon analysis

Troy

Kable

Schmidt

et al. 2012

A&A

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“…Starting from the geometry of each fully hydrogenated PAH (Malloci et al 2007a), we considered its dehydrogenated derivatives. We considered only neutral and cationic charge-states, and found in all cases that the computed ground-states for neutrals and cations are the singlet and the doublet, respectively.…”

Section: Computational Detailsmentioning

confidence: 99%

“…Keeping fixed the ground-state optimised geometries obtained above, we then computed the absorption cross-section σ(E) using the real-time TD-DFT implementation of octopus (Marques et al 2003), in the local-density approximation (Perdew & Zunger 1981) as described in Malloci et al (2007a). We previously showed (Malloci et al 2004;Mulas et al 2006) that the results of this method reproduce the overall far-UV behaviour of σ(E), as seen in comparison with the experimental data available for a few neutral PAHs (Joblin 1992;Joblin et al 1992).…”

Section: Computational Detailsmentioning

confidence: 99%

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

Malloci

Mulas

Cecchi‐Pestellini

et al. 2008

A&A

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Context. Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of polycyclic aromatic hydrocarbons (PAHs) in several ionisation states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C 24 H n , n ≤ 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. Aims. To assess the observational consequences of the latter hypothesis we have undertaken a systematic theoretical study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. Methods. We performed calculations using state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time-dependent DFT to evaluate the electronic excited-state properties. Results. We computed the absorption cross-section of the species C 24 H n (n = 12, 10, 8, 6, 4, 2, 0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fully dehydrogenated counterparts. Conclusions. π-electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the π → π * collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.

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“…This model takes advantage of the latest available molecular data from both theoretical calculations (Malloci et al 2007(Malloci et al , 2008 and experimental measurements (Biennier et al 2006;Betts et al 2006;Joblin et al, in prep.). We apply our model to the prototypical north-west PDR (hereafter NW PDR) of NGC 7023 in an attempt to (i) identify key processes in the evolution of the charge and hydrogenation states of interstellar PAHs in order to provide guidelines for fundamental studies and to (ii) provide a quantitative description of the evolution of the PAH population, which is necessary for modelling the role of PAHs in the physics and chemistry of PDRs.…”

Section: Introductionmentioning

confidence: 99%

Evolution of polycyclic aromatic hydrocarbons in photodissociation regions

Montillaud

Joblin

Toublanc

2013

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Context. Various studies have emphasised variations in the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). These changes are expected to affect the energetics and chemistry in these regions, thereby calling for a quantitative description. Aims. We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023, for which many observational constraints are available. We also discuss the case of the diffuse interstellar medium (ISM). Methods. We modelled the physical conditions in NGC 7023 NW using a state-of-the-art PDR code. We then used a new PAH chemical evolution model that includes recent experimental data on PAHs and describes multiphoton events. We considered a family of compact PAHs bearing up to 96 carbon atoms. Results. The calculated ionization ratio is in good agreement with the observed ratio in NGC 7023 NW. Within the PDR, PAHs evolve into three major populations. We find medium-sized PAHs (50 N C 90) to be normally hydrogenated, while larger PAHs (N C 90) can be superhydrogenated, and smaller species (N C 50) are fully dehydrogenated. In the more diffuse gas of the cavity, where the fullerene C 60 has recently been detected, all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical evolution exhibits a complex non-linear behaviour as a function of the UV radiation field because of multiphoton events. Steady state for hydrogenation is reached on timescales ranging from less than a year for small PAHs, up to 10 4 years for large PAHs at A V = 1. Critical reactions that would need more studies are the recombination of cations with electrons, the reactivity of cations with H 2 , and the reactivity of neutral PAHs with H. Conclusions. We have developed a new model of PAH chemical evolution based on the most recent available molecular data. This model allows us to rationalise the observational constraints without any fitting parameter. PAHs smaller than 50 carbon atoms are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is obtained for the diffuse ISM. Carbon clusters turn out to be end products of PAH photodissociation, and the evolution of these clusters needs to be investigated further to evaluate their impact on the chemical and physical evolution of PDRs.

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Theoretical evaluation of PAH dication properties

Cited by 74 publications

References 89 publications

On the electronic spectroscopy of closed-shell cations derived from resonance-stabilized radicals: Insights from theory and Franck-Condon analysis

On the electronic spectroscopy of closed-shell cations derived from resonance-stabilized radicals: Insights from theory and Franck-Condon analysis

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

Evolution of polycyclic aromatic hydrocarbons in photodissociation regions

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