Effects of hydrogen dissociation on the infrared emission spectra of naphthalene: theoretical modeling

Falvo, Cyril; Friha, Hela; Pino, T.; Dhaouadi, Zoubeida; Parneix, P.; Calvo, F.; Bréchignac, Philippe

doi:10.1039/c3cp44703k

Cited by 6 publications

(8 citation statements)

References 37 publications

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“…Only few pioneering studies of this nature exist in the literature, mostly limited to the availability of QFF for PAHs. These represnt the clear starting point for a more exhaustive treatment of the emission process and photophysics of PAHs molecules in space, including isomerisation and dissociation events …”

Section: Toward a Full Modeling Of The Pah Emissionmentioning

confidence: 99%

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Anharmonic interstellar PAH molecules

Candian

Mackie

2016

Int J of Quantum Chemistry

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Polycyclic aromatic hydrocarbon (PAH) molecules are responsible for a family of features, the aromatic infrared bands, which are observed in many astronomical environments. The photophysics and photochemistry of PAHs in space is strongly influenced by anharmonicity, which has seldom been considered in astrophysical modeling because of the lack of available data or because too computationally demanding. Recent developments of second-order vibrational perturbation theory applied to density functional theory now allow the computation of the anharmonic quartic force field anharmonic spectra of medium-large molecules like PAHs in a time-efficient way. It is now possible to have a better grasp of the effects of anharmonicity on the vibrational spectrum of PAHs also thanks to the comparison with gas-phase, high-resolution experimental data. In this perspective, the quality of these anharmonic spectra for PAH molecules is reviewed and discussed in view of a robust astrochemical model for PAH molecules.anharmonic, density functional theory, polycyclic aromatic hydrocarbons | I N T R O D U C T I O NPolycyclic aromatic hydrocarbon (PAH) are a class of molecules which is very common on the Earth, being the byproduct of combustion. It is now known that PAHs are widespread in the entire Universe. They are accepted almost unequivocally as the carriers of a family of bands, the aromatic infrared bands (AIBs), detected in emission in the spectrum of astronomical objects ranging from dying stars to entire galaxies. [1] In space, PAH molecules absorb ultraviolet or visible photons, then undergo fast internal conversion by which the absorbed energy is transferred to the vibrational degrees of freedom. The energy is then released through a radiative cascade of the vibrational transitions down to the ground state, producing the AIBs. [2] As the average time for IR photon emission is much longer than the time for internal vibrational redistribution, the population of the vibrational states is in internal equilibrium, leading to emission between excited vibrational states rather than only fundamentals. Thus, the appearance of the AIBs spectrum (i.e., band position, band width, and rovibrational envelope) is strongly influenced by the anharmonicity of the potential energy surface of the molecules. The profile and general appearance of the AIB spectrum has been probed by multiple astronomical observations. TheAIBs not only show variation between different astronomical objects [3] but also spatially across a given single astronomical object. [4] This has led astronomers to hypothesize that these spectral variations are linked to the variation of local physical conditions which, in turn, affects the PAH population. The spectroscopy of PAH molecules is difficult to study experimentally in condition relevant to space due to their low vapor pressure; computational chemistry thus has been the major source of vibrational spectra. Databases exist containing the harmonic vibrational spectra of many different PAHs, up to molecules containi...

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Section: Toward a Full Modeling Of The Pah Emissionmentioning

confidence: 99%

“…These represnt the clear starting point for a more exhaustive treatment of the emission process and photophysics of PAHs molecules in space, including isomerisation and dissociation events. [29]…”

mentioning

confidence: 99%

Anharmonic interstellar PAH molecules

Candian

Mackie

2016

Int J of Quantum Chemistry

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“…In this context, there is a need for systematic computation of the finite temperature spectra of PAHs including the methylenepyrene cation. Previous studies have already been performed along these lines on a few specific PAHs and PAH-derived species. − The search for the carriers of the DIBs. In that respect, many theoretical electronic spectra using the TD-DFT method have been determined for closed shell or radical molecular ions derived from PAHs over the past 15 years. − However, experimental studies to determine electronic spectra using, for instance, the MPD technique involve finite temperature conditions.…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Cationic Methylene–Pyrene Isomers and Isomerization Pathways: Finite Temperature Theoretical Studies

et al. 2015

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This paper provides spectral characterizations of the two isomers of the 1-methylenepyrene cation, namely, the 1-pyrenemethylium and a pyrene-like isomer owing a tropylium cycle. Both are possible photodissociation products of the 1-methylpyrene cation and were proposed as potential contributors to the diffuse interstellar bands. In that respect, vibrational and electronic spectra are computed for the optimized structures at the density functional theory (DFT) and time-dependent (TD-)DFT levels. Finite temperature effects on these spectra are estimated from molecular dynamics simulations within the density functional-based tight-binding (DFTB) and TD-DFTB frameworks, these methods being first benchmarked against DFT and TD-DFT calculations. The computed spectra allow discrimination of the two isomers. When the temperature increases, bands are observed to redshift and merge. The isomerization mechanism is investigated with the metadynamics technique, a biased dynamics scheme allowing to probe reaction mechanisms with high energy barriers by investigating the free energy surface at various temperatures. Four pathways with similar barrier heights (3.5-4 eV) are found, showing that the interconversion process would only occur in interstellar clouds under photoactivation. The present study opens the way to simulations on larger methyl- and methylenePAHs of astrophysical interest and their experimental investigation.

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“…In fact, one of the reasons for the identification problem may come from the competition between IR emission, isomerisation and dissociation (see ref. [12] for instance). More generally, the investigation of energetic processing of astroPAHs is currently an important research topic as it can significantly impact the physics and chemistry of the ISM [44].…”

Section: Introductionmentioning

confidence: 99%

Dissociation of polycyclic aromatic hydrocarbons at high energy: MD/DFTB simulations versus collision experiments

et al. 2018

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The whole process following collisions of polycyclic aromatic hydrocarbons (PAHs) with high energetic protons is modeled and compared to the experimental mass spectrum, allowing to propose a coherent scenario. Fragmentation of cationic pyrene C 16 H + 10 is extensively studied by molecular dynamics simulations obtained by computing the electronic structure at the Self-Consistent-Charge Density Functional based Tight Binding (MD/SCC-DFTB) on-the-fly. An atomic model is used to quantify the energy transfered to the target after proton impact, and assuming fast internal conversion for the produced cations. From this model, after ionisation, the molecules show a broad distribution of internal energy with a rough exponential decrease. This distribution is used as an input for further extensive MD/SCC-DFTB simulations. The good agreement between experimental and theoretical spectra globally validates the SCC-DFTB potential, the wide distribution of fragments corresponding to statistical dissociation. The scenario for both the internal energy deposited distribution and the fast internal conversion assumption is validated. Using these assumptions, dissociation is shown to occur within a few hundreds of picoseconds. Moreover, adjusting the experimental mass spectrum with the theoretical spectra obtained for the various internal energies nicely returns the distribution modeled from the atomic contributions, reinforcing the coherence of the global approach. This study lays the foundations for further synergistic theoretical and experimental studies that will be devoted to other PAHs and prebiotic molecules of astrophysical interest.

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Effects of hydrogen dissociation on the infrared emission spectra of naphthalene: theoretical modeling

Cited by 6 publications

References 37 publications

Anharmonic interstellar PAH molecules

Anharmonic interstellar PAH molecules

Cationic Methylene–Pyrene Isomers and Isomerization Pathways: Finite Temperature Theoretical Studies

Dissociation of polycyclic aromatic hydrocarbons at high energy: MD/DFTB simulations versus collision experiments

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