The Nasa Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database: The Computed Spectra

Bauschlicher, Charles W.; Boersma, C.; Ricca, Alessandra; Mattioda, A. L.; Cami, J.; Peeters, E.; Armas, F. Sánchez de; Saborido, G. Puerta; Hudgins, Douglas M.; Allamandola, Louis J.

doi:10.1088/0067-0049/189/2/341

Cited by 180 publications

(165 citation statements)

References 45 publications

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“…Mixtures of PAHs, aromatic-aliphatic molecules and hyperhydrogenated PAHs molecules have been proposed to account for the astronomical infrared observations 41 . All these aromatic chemical species can be produced out of the above-presented processing mechanism of the SiC dust.…”

Section: Discussionmentioning

confidence: 99%

Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

et al. 2014

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Polycyclic aromatic hydrocarbons as well as other organic molecules appear among the most abundant observed species in interstellar space and are key molecules to understanding the prebiotic roots of life. However, their existence and abundance in space remain a puzzle. Here we present a new top-down route to form polycyclic aromatic hydrocarbons in large quantities in space. We show that aromatic species can be efficiently formed on the graphitized surface of the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure and temperature conditions analogous to those of the interstellar medium. To this aim, we mimic the circumstellar environment using ultra-high vacuum chambers and investigate the SiC surface by in situ advanced characterization techniques combined with first-principles molecular dynamics calculations. These results suggest that top-down routes are crucial to astrochemistry to explain the abundance of organic species and to uncover the origin of unidentified infrared emission features from advanced observations.

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Section: Discussionmentioning

confidence: 99%

Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

et al. 2014

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“…A&A 530, A26 (2011) The vibronic progressions established for several DIBs (Duley & Kuzmin 2010) point toward an origin from a torsional motion of pendent rings, suggesting that the carriers are "floppy" polycyclic aromatic hydrocarbon (PAH) structures, such as 9-phenylanthracene, parasubstituted biphenyls, tetracene derivatives, bianthracene, or tolane, rather than compact molecules. The presence of PAHs in the interstellar medium is now generally established, and there is unequivocal evidence that the unidentified near-infrared emission bands arise from PAHs (Leger & Puget 1984;Allamandola et al 1985;Tielens 2008;Bauschlicher et al 2010). Mixtures of PAHs or nanosized hydrogenated dust particles have been proposed to explain the 2175 Å UV bump seen toward stars reddened by translucent material (Joblin et al 1992;Mennella et al 1998;Steglich et al 2010), and the presence of carbonaceous dust particles in translucent material is well established (Henning & Salama 1998).…”

Section: Introductionmentioning

confidence: 99%

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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“…These very weak features are severely blended with much stronger H 2 O ice bands, consistent with the number of PAH molecules relative to the number of H 2 O molecules along these lines of sight on the order of a few percent. So far, it has proven difficult to unambiguously interpret these absorption features in spite of the fact that there is a growing database of theoretically calculated and laboratory measured IR absorption spectra of both neutral and ionized PAHs in inert matrices (e.g., Szczepanski & Vala 1993;Szczepanski et al 1993aSzczepanski et al ,b, 1995aHudgins et al 1994;Hudgins & Allamandola 1995aLanghoff 1996;Mattioda et al 2005b;Bauschlicher et al 2009Bauschlicher et al , 2010, and references therein). Unfortunately, these spectra cannot be used directly to compare with PAHs in H 2 O-rich ices, as rare gas matrix spectra will be different.…”

Section: Introductionmentioning

confidence: 99%

Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice

Bouwman

Mattioda

Linnartz

et al. 2010

A&A

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Context. Polycyclic aromatic hydrocarbons (PAHs) are known to be abundantly present in photon-dominated regions (PDRs), as evidenced by their ubiquitous mid-IR emission bands. Towards dense clouds, however, their IR emission bands are strongly suppressed. It is here where molecules are known to reside on very cold grains (T ≤ 30 K) in the form of interstellar ices. Therefore, it is likely that non-volatile species, such as PAHs, also freeze out on grains. Such icy grains act as catalytic sites and, upon vacuum ultraviolet (VUV) irradiation, chemical reactions are initiated. In the study presented here, these reactions and the resulting photoproducts are investigated for PAH containing water ices. Aims. The aim of this work is to monitor vacuum ultraviolet induced chemical reactions of PAHs in cosmic ice through their IR signatures, to characterize the families of species formed in these reactions, and to apply the results to astronomical observations. Methods. Mid-infrared Fourier transform absorption spectroscopic measurements ranging from 6500 to 450 cm −1 are performed on freshly deposited and vacuum ultraviolet processed PAH containing cosmic H 2 O ices at low temperatures. Results. The mid-IR spectroscopy of anthracene, pyrene and benzo[ghi]perylene containing H 2 O ice is reported. Band strengths of the neutral PAH modes in H 2 O ice are derived. Additionally, spectra of vacuum ultraviolet processed PAH containing H 2 O ices are presented. These spectra are compared to spectra measured in VUV processed PAH:argon matrix isolation studies. It is concluded that the parent PAH species is ionized in H 2 O ice and that other photoproducts, mainly more complex PAH derivatives, also form. The importance of PAHs and their PAH:H 2 O photoproducts in astronomical mid-infrared spectroscopic studies, in particular in the 5−8 μm region, is discussed. As a test-case, the VUV photolyzed PAH:H 2 O laboratory spectra are compared to a high resolution ISO-SWS spectrum of the high-mass embedded protostar W33A and to a Spitzer spectrum of the low-mass Young Stellar Object (YSO) RNO 91. For these objects, an upper limit of 2-3% with respect to H 2 O ice is derived for the contribution of PAHs and PAH:H 2 O photoproducts to the absorbance in the 5−8 μm region towards these objects.

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The Nasa Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database: The Computed Spectra

Cited by 180 publications

References 45 publications

Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

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

Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice

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