Electronic Spectroscopy of PAHs

Pino, T.; Carpentier, Y.; Féraud, Géraldine; Friha, Hela; Kokkin, Damian L.; Troy, Tyler P.; Chalyavi, Nahid; Bréchignac, Ph.; Schmidt, Timothy W.

doi:10.1051/eas/1146037

Cited by 13 publications

(12 citation statements)

References 64 publications

(57 reference statements)

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“…However, for coronene, mean fluorescence and phosphorescence yields are of only 0.2 and 0.1, respectively (Dawson & Kropp 1969). Because of smaller energy gaps, fluorescence and phosphorescence for large species is unimportant (Pino et al 2011). For ions, internal conversion will be even more important as the energy gaps involved are smaller (cf.…”

Section: Photodissociation Channelsmentioning

confidence: 99%

“…For ions, internal conversion will be even more important as the energy gaps involved are smaller (cf. Pino et al 2011). Delayed fluorescence -also sometimes called Poincaré fluorescence, where the excited species revisits the S 1 (or T 1 ) state (Léger et al 1988) -is measured to be unimportant for coronene as it only accounts for a small fraction of the total radiationless de-activation of triplet coronene (Kropp & Dawson 1967).…”

Section: Photodissociation Channelsmentioning

confidence: 99%

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Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation

Andrews

Candian

Tielens

2016

A&A

117

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Context. We have modelled the abundance distribution and IR emission of the first 3 members of the coronene family in the north-west photodissociation region of the well-studied reflection nebulae NGC 7023. Aims. Our aim was 3-fold: i) analyze the distribution of abundances; (ii) examine the spectral footprints from the hydrogenation state of polycyclic aromatic hydrocarbons (PAHs); and (iii) assess the role of PAHs in the formation of H 2 in photodissociation regions. Methods. To model the physical conditions inside the cloud, we used the Meudon PDR Code, and we gave this as input to our kinetic model. We used specific molecular properties for each PAH, based on the latest data available at the present time. We considered the loss of an H atom or an H 2 molecule as multiphoton processes, and we worked under the premise that PAHs with extra H atoms can form H 2 through an Eley-Rideal abstraction mechanism. Results. In terms of abundances, we can distinguish clear differences with PAH size. The smallest PAH, coronene (C 24 H 12 ), is found to be easily destroyed down to the complete loss of all of its H atoms. The largest species circumcircumcoronene (C 96 H 24 ), is found in its normal hydrogenated state. The intermediate size molecule, circumcoronene (C 54 H 18 ), shows an intermediate behaviour with respect to the other two, where partial dehydrogenation is observed inside the cloud. Regarding spectral variations, we find that the emission spectra in NGC 7023 are dominated by the variation in the ionization of the dominant hydrogenation state of each species at each point inside the cloud. It is difficult to "catch" the effect of dehydrogenation in the emitted PAH spectra since, for any conditions, only PAHs within a narrow size range will be susceptible to dehydrogenation, being quickly stripped off of all H atoms (and may isomerize to cages or fullerenes). The 3 µm region is the most sensitive one towards the hydrogenation level of PAHs. Conclusions. Based on our results, we conclude that PAHs with extra H atoms are not the carriers of the 3.4 µm band observed in NGC 7023, since these species are only found in very benign environments. Finally, concerning the role of PAHs in the formation of H 2 in photodissociation regions, we find that H 2 abstraction from PAHs with extra H atoms is an inefficient process compared to grains. Instead, we propose that photodissociation of PAHs of small-to-intermediate sizes could contribute to H 2 formation in PDR surfaces, but they cannot account by themselves for the inferred high H 2 formation rates in these regions.

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Section: Photodissociation Channelsmentioning

confidence: 99%

Section: Photodissociation Channelsmentioning

confidence: 99%

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation

Andrews

Candian

Tielens

2016

A&A

117

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“…Some relevant experimental data is summarized in Figure 3 (Pino et al 2011), illustrating the energy gap law (derived from measured linewidths) but similar results also apply to hydrocarbon chains (Ding et al 2003;Boguslavskiy et al 2005;Boguslavskiy & Maier The Way Forward 405 2006). Thus, for visible transitions where the energy gap is 0.5 − 1 eV (eg., typically one or more lower lying, excited electronic states), the line width will be some 20Å.…”

Section: Spectroscopy Of Dib Carriersmentioning

confidence: 64%

“…A number of research groups have made a dedicated effort to study relevant DIB carriers in the laboratory: notably John Maier and his group in Basel have focused on the spectroscopic characteristics of hydrocarbon chains, while Farid Salama and his group at NASA Ames, Huiskens and his group at Jena, and Thomas Pino and Philippe Brechignac at the IAS have put the emphasis on the PAH molecules (Maier et al 2004Salama et al 2011;Gredel et al 2011;Pino et al 2011). These efforts are reviewed elsewhere in this volume.…”

Section: Spectroscopy Of Dib Carriersmentioning

confidence: 99%

Diffuse Interstellar Bands: The Way Forward

Tielens¹

2013

Proc. IAU

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Abstract. Rather than a summary of the conference, I present here an overview of the status of the field and our progress over the last two decades from the points of view of astronomy, molecular physics, spectroscopy, and astrochemistry. While at first sight, progress may seem slow, actually, we have made an important stride forward. We have recognized now that the problem is very complex and identifying the carriers of the Diffuse Interstellar Bands will require a concerted effort of astronomers, molecular physicists, spectroscopists, and astrochemists. While this is a daunting prospect, we have identified the tools that we need to make this happen.

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“…Recent studies have focused on obtaining laboratory spectroscopic data of low temperature gas-phase neutral PAHs to compare with astronomical spectra of diffuse and translucent clouds (e.g. Pino et al (2011), Dietsche et al (2011). Ten different PAHs (containing 2 to 13 benzene rings / 11 to 42 carbon atoms) were searched for but none were detected in the ISM by both and Gredel et al (2011) independently.…”

Section: Dib Carrier Candidatesmentioning

confidence: 99%

The Diffuse Interstellar Bands: an Elderly Astro-Puzzle Rejuvenated

Cox

2011

Proc. IAU

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Abstract. The interstellar medium constitutes a physically and chemically complex component of galaxies and is important in the cycle of matter and the evolution of stars. From various spectroscopic clues we now know that the interstellar medium is rich in organic compounds. However, identifying the exact nature of all these components remains a challenge. In particular the identification of the so-called diffuse band carriers has been alluding astronomers for almost a century.In recent decades, observational, experimental and theoretical advances have rapidly lead to renewed interest in the diffuse interstellar bands (DIBs). This has been instigated partly by their perceived relation to the infrared aromatic emission bands, the UV extinction bump and far-UV rise, and the growing number of (small) organic molecules identified in space.This chapter gives an overview of the observational properties and behaviour of the DIBs, and their presence throughout the Universe. I will highlight recent progress in identifying their carriers and discuss their potential as tracers and probes of (extra)-Galactic ISM conditions.

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Electronic Spectroscopy of PAHs

Cited by 13 publications

References 64 publications

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation

Diffuse Interstellar Bands: The Way Forward

The Diffuse Interstellar Bands: an Elderly Astro-Puzzle Rejuvenated

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Electronic Spectroscopy of PAHs

Cited by 13 publications

References 64 publications

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

Diffuse Interstellar Bands: The Way Forward

The Diffuse Interstellar Bands: an Elderly Astro-Puzzle Rejuvenated

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Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H₂formation