The Abundances of Hydrocarbon Functional Groups in the Interstellar Medium Inferred From Laboratory Spectra of Hydrogenated and Methylated Polycyclic Aromatic Hydrocarbons

Steglich, Mathias; Jäger, Cornelia; Huisken, F.; Friedrich, M.; Plass, Winfried; Räder, Hans‐Joachim; Müllen, Kläus; Henning, Thomas

doi:10.1088/0067-0049/208/2/26

Cited by 46 publications

(40 citation statements)

References 62 publications

(119 reference statements)

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“…A final feature of interest is the 3.40-µm feature on the 3-µm plateau. Several explanations have been put forward to account for its presence ranging from hot bands of aromatic CH-stretch transition υ = 2 → 1 shifted due to anharmonic effects (Barker et al 1987) to CH-stretch modes in methylated (Joblin et al 1996;Pauzat et al 1999) and superhydrogenated (Bernstein et al 1996;Steglich et al 2013;Sandford et al 2013;Wagner et al 2000) PAHs , but none of these have so far been confirmed. The overall picture that emanates from our high-resolution studies, involving in particular the role of anharmonicity, suggests that an explanation based on hydrogenated and alkylated PAHs is attractive.…”

Section: Astrophysical Implicationsmentioning

confidence: 99%

HIGH-RESOLUTION IR ABSORPTION SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE 3 μm REGION: ROLE OF PERIPHERY

Maltseva

Petrignani

Candian

et al. 2016

ApJ

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In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3 μm absorption band. For this purpose, we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz [a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 cm −1 range. The experimental spectra are compared with standard harmonic calculations and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3 μm region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive data set of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3 μm band, and on features such as the two-component emission character of this band and the 3 μm emission plateau.

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Section: Astrophysical Implicationsmentioning

confidence: 99%

HIGH-RESOLUTION IR ABSORPTION SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE 3 μm REGION: ROLE OF PERIPHERY

Maltseva

Petrignani

Candian

et al. 2016

ApJ

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“…In many interstellar and circumstellar environments the 3.3 µm emission feature is indeed often accompanied by a weaker feature at 3.4 µm (see Li & Draine 2012 and references therein). As demonstrated by Li & Draine (2012) and Yang et al (2013), one can place an upper limit of ∼ 2% on the aliphatic fraction of the emitters of the UIE features by assigning the 3.4 µm emission exclusively to aliphatic C-H (also see Rouillé et al 2012, Steglich et al 2013. This is indeed an upper limit as the 3.4 µm emission feature could also be due to anharmonicity of the aromatic C-H stretch (Barker et al 1987) and "superhydrogenated" PAHs whose edges contain excess H atoms (Bernstein et al 1996, Sandford et al 2013.…”

mentioning

confidence: 95%

On the aliphatic versus aromatic content of the carriers of the ‘unidentified’ infrared emission features

Yang¹,

Glaser²,

Li³

et al. 2016

Mon. Not. R. Astron. Soc.

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Although it is generally accepted that the unidentified infrared emission (UIE) features at 3. 3, 6.2, 7.7, 8.6, and 11.3 µm are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, the exact nature of their carriers remains unknown: whether they are free-flying, predominantly aromatic gas-phase molecules, or amorphous solids with a mixed aromatic/aliphatic composition are being debated. Recently, the 3.3 and 3.4 µm features which are commonly respectively attributed to aromatic and aliphatic C-H stretches have been used to place an upper limit of ∼ 2% on the aliphatic fraction of the UIE carriers (i.e., the number of C atoms in aliphatic chains to that in aromatic rings). Here we further explore the aliphatic versus aromatic content of the UIE carriers by examining the ratio of the observed intensity of the 6.2 µm aromatic C-C feature (I 6.2 ) to that of the 6.85 µm aliphatic C-H deformation feature (I 6.85 ). To derive the intrinsic oscillator strengths of the 6.2 µm stretch (A 6.2 ) and the 6.85 µm deformation (A 6.85 ), we employ density functional theory to compute the vibrational spectra of seven methylated polycyclic aromatic hydrocarbon molecules and their cations. By comparing I 6.85 /I 6.2 with A 6.85 /A 6.2 , we derive the fraction of C atoms in methyl(ene) aliphatic form to be at most ∼ 10%, confirming the earlier finding that the UIE emitters are predominantly aromatic. We have also computed the intrinsic strength of the 7.25 µm feature (A 7.25 ), another aliphatic C-H deformation band. We find that A 6.85 appreciably exceeds A 7.25 . This explains why the 6.85 µm feature is more frequently detected in space than the 7.25 µm feature.

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“…In recent years, the aliphatic fraction of the UIE carriers -the ratio of the number of C atoms in aliphatic units (N C,ali ) to that in aromatic rings (N C,aro ) -has received much attention (e.g., see Kwok & Zhang 2011, Li & Draine 2012, Rouillé et al 2012, Steglich et al 2013, Yang et al 2013, 2016a. Kwok & Zhang (2011) argued that the material responsible for the UIE features may have a substantial aliphatic component and therefore, by definition, PAHs can not be the UIE carrier.…”

Section: Introductionmentioning

confidence: 99%

Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C–H Stretching Modes and Astrophysical Implications

Yang

Glaser

et al. 2017

ApJ

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The so-called unidentified infrared emission (UIE) features at 3. 3, 6.2, 7.7, 8.6, and 11.3 µm ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs may have an aliphatic component as revealed by the detection in many UIE sources of the aliphatic C-H stretching feature at 3.4 µm. The ratio of the observed intensity of the 3.4 µm feature to that of the 3.3 µm aromatic C-H feature allows one to estimate the aliphatic fraction of the UIE carriers. This requires the knowledge of the intrinsic oscillator strengths of the 3.3 µm aromatic C-H stretch (A 3.3 ) and the 3.4 µm aliphatic C-H stretch (A 3.4 ). Lacking experimental data on A 3.3 and A 3.4 for the UIE candidate materials, one often has to rely on quantum-chemical computations. Although the second-order Møller-Plesset (MP2) perturbation theory with a large basis set is more accurate than the B3LYP density functional theory, MP2 is computationally very demanding and impractical for large molecules. Based on methylated PAHs, we show here that, by scaling the band strengths computed at an inexpensive level (e.g., B3LYP/6-31G * ) we are able to obtain band strengths as accurate as that computed at far more expensive levels (e.g., MP2/6-311+G(3df,3pd)). We calculate the model spectra of methylated PAHs and their cations excited by starlight of different spectral shapes and intensities. We find (I 3.4 /I 3.3 ) mod , the ratio of the model intensity of the 3.4 µm feature to that of the 3.3 µm feature, is insensitive to the spectral shape and intensity of the exciting starlight. We derive a straightforward relation for determining the aliphatic fraction of the UIE carriers (i.e., the ratio of the number of C atoms in aliphatic units N C,ali to that in aromatic rings N C,aro ) from the observed band ratios (I 3.4 /I 3.3 ) obs : N C,ali /N C,aro ≈ 0.57 × (I 3.4 /I 3.3 ) obs for neutrals and N C,ali /N C,aro ≈ 0.26 × (I 3.4 /I 3.3 ) obs for cations.

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The Abundances of Hydrocarbon Functional Groups in the Interstellar Medium Inferred From Laboratory Spectra of Hydrogenated and Methylated Polycyclic Aromatic Hydrocarbons

Cited by 46 publications

References 62 publications

HIGH-RESOLUTION IR ABSORPTION SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE 3 μm REGION: ROLE OF PERIPHERY

HIGH-RESOLUTION IR ABSORPTION SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE 3 μm REGION: ROLE OF PERIPHERY

On the aliphatic versus aromatic content of the carriers of the ‘unidentified’ infrared emission features

Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C–H Stretching Modes and Astrophysical Implications

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