The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing Small Molecular Polycyclic Aromatic Hydrocarbons

Clayton, Geoffrey C.; Salama, Farid; Allamandola, Louis J.; Martín, P. G.; Snow, Theodore P.; Whittet, D. C. B.; Witt, A. N.; Wolff, Michael

doi:10.1086/375771

Cited by 80 publications

(73 citation statements)

References 47 publications

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“…The predominance of thermionic emission across all PE spectra is similar to that recently characterized for the radical anion of menadione, and suggests that C 18 H 11 -is resilient to direct photodetachment by broadband UV radiation. 26,[69][70][71][72][73][74] Similarly, other results from our laboratory and from the Bordas group indicate that thermionic emission dominates the PE spectroscopy of C 60 -and other fullerene anions. 37,75 Such efficient ground state recovery dynamics is probably a generic feature of PAH anions, and would support their existence in favourable astrochemical environments.…”

Section: Thermionic Emissionsupporting

confidence: 68%

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Bull

West

Verlet

2015

Phys. Chem. Chem. Phys.

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Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuumJames N. Bull a , Christopher W. West a and Jan R. R. Verlet a † Photoelectron velocity-map imaging and electronic structure calculations have been used to study the temporary anion (resonance) dynamics of the closed-shell site-specific deprotonated tetracene anion (C18H11 -) in the hv = 3.26 eV (380 nm) to 4.13 eV (300 nm) range. In accord with a recent frequency-, angle-, and time-resolved photoelectron imaging study on a related but open-shell polyaromatic radical anion (Chem. Sci., 2015, 6, 1578-1589, population of π*-resonances situated in the detachment continuum efficiently recover the ground electronic state of the anion through ultrafast non-adiabatic dynamics, followed by characteristic statistical electron loss (thermionic emission). The combined electron yield of direct photodetachment and autodetachment from the optically-accessed resonances in C18H11 -is several orders of magnitude smaller than thermionic emission from the ground electronic electronic state in the photon energy range studied. This result implies a resiliance to prompt photoejection from UV radiation, and the ability of neutral PAH-like species to capture a free electron and form a long-lived molecular anion that ultimately decays by thermionic emission on a millisecond timescale. The attachment mechanism applies to polyaromatic species that cannot support dipole-bound states, and may provide an additional route to forming anions in astrochemical environments. IntroductionPolyaromatic hydrocarbon (PAH) molecules are found across a diverse range of chemical environments. For example, they form as products in combustion and anthropogenic pollution, 1 they are used in new generation semiconductors and microelectronics, 2,3 and they have been postulated to be present in astrochemical environments. 4,5 In astrochemistry, anions are recognised as important chemical species, [6][7][8][9][10][11] and have been identified as small polyynes such as C 4 H -and C 6 H -, [12][13][14][15][16][17][18] and are commonly postulated to be present as PAHs. However, the detailed formation mechanism of astrochemical anions is unclear. It is commonly postulated that stable anions may be formed via electric dipole-bound doorway states; 19,20 however, most neutral PAH molecules do not have a sufficient dipole moment to support such a mechanism.Most molecules,...

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Section: Thermionic Emissionsupporting

confidence: 68%

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Bull

West

Verlet

2015

Phys. Chem. Chem. Phys.

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“…Lewis et al (1989) suggested that it would be compatible with the measured interstellar extinction (e.g. Stecher 1965; Fitzpatrick & Massa 1986;Clayton et al 2003), if <10% of the carbon would be present as diamonds. Interestingly, the comparison of mass absorption coefficient spectra in Fig.…”

Section: Discussionmentioning

confidence: 75%

Optical data of meteoritic nano-diamonds from far-ultraviolet to far-infrared wavelengths

Mutschke

Andersen

Jäger

et al. 2004

A&A

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Abstract.We have used different spectroscopic techniques to obtain a consistent quantitative absorption spectrum of a sample of meteoritic nano-diamonds in the wavelength range from the vacuum ultraviolet (0.12 µm) to the far infrared (100 µm). The nano-diamonds have been isolated by a chemical treatment from the Allende meteorite (Braatz et al. 2000, Meteorit. Planet. Sci., 35, 75). Electron energy loss spectroscopy (EELS) extends the optical measurements to higher energies and allows the derivation of the optical constants (n & k) by Kramers-Kronig analysis. The results can be used to restrain observations and to improve current models of the environment where the nano-diamonds are expected to have formed. We also show that the amount of nano-diamond which can be present in space is higher than previously estimated by Lewis et al. (1989, Nature, 339, 117).

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“…In their investigation of the ultraviolet interstellar extinction curve using the HST/STIS spectrograph, Clayton et al (2003) did not detect absorption bands in the 1150−3180 Å wavelength region that could be associated with PAHs. The authors concluded that either PAHs are destroyed by the interstellar radiation field, or that they occur in such a large variety that individual absorptions are too weak to be detected.…”

Section: Fractional Pah Abundances In Translucent Molecular Cloudsmentioning

confidence: 96%

Abundances of PAHs in the ISM: confronting observations with experimental results

Gredel

Carpentier

Rouillé

et al. 2011

A&A

104

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Context. The identification of the carriers of the diffuse interstellar bands (DIBs) is the longest standing problem in the study of the interstellar medium. Here we present recent UV laboratory spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. Whereas, in the near IR range, the PAHs exhibit vibrational bands that are not molecule-specific, their electronic transitions occurring in the UV/vis provide characteristic fingerprints. The comparison of laboratory spectra calibrated in intensity with high signal-to-noise observational data in the UV enables us to establish new constraints on PAH abundances. Aims. From a detailed comparison of the gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we aim to infer the abundance of these PAHs in the interstellar medium. Methods. We present spectra of PAHs measured at low temperature in the gas phase and in an Ne matrix, and present methods to derive absolute absorption cross sections for the matrix and gas phase spectra. We have obtained high signal to noise (S/N > 100) absorption spectra toward five lines of sight with reddenings of E B−V = 1−1.6 mag. The spectra cover the 3050−3850 Å wavelength region where the PAHs studied here show prominent absorption features. Results. From the observations, we infer upper limits in the fractional abundances of the PAHs studied here. Upper limits in the column densities of anthracene of 0.8−2.8 × 10 12 cm −2 and of pyrene and 2,3-benzofluorene ranging from 2−8 × 10 12 cm −2 are inferred. Upper limits in the column densities of benzo [ghi]perylene are 0.9−2.4 × 10 13 and 10 14 cm −2 for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times 10 −10 . Upper limits in the fractional abundance of benzo [ghi]perylene of a few times 10 −9 and of phenanthrene of few times 10 −8 are inferred. Toward CPD −32 • 1734, we found near 3584 Å an absorption line of OH + , which was discovered in the interstellar medium only very recently. Conclusions. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds or that a PAH population with a wide variety of molecules is present.

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The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing Small Molecular Polycyclic Aromatic Hydrocarbons

Cited by 80 publications

References 47 publications

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Optical data of meteoritic nano-diamonds from far-ultraviolet to far-infrared wavelengths

Abundances of PAHs in the ISM: confronting observations with experimental results

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