Single‐Photon Infrared Emission Spectroscopy of Gaseous Polycyclic Aromatic Hydrocarbon Cations: A Direct Test for Proposed Carriers of the Unidentified Infrared Emission Bands

Kim, Hack Sung; Saykally, Richard J.

doi:10.1086/343078

Cited by 54 publications

(41 citation statements)

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“…Experimentally, IR spectra of PAH neutrals, cations, and other derivatives have been measured in rare gas matrices (e.g., Szczepanski & Vala 1993a, 1993bHudgins & Allamandola 1995a, 1995bMattioda et al 2003;Bernstein et al 2007;Tsuge et al 2016). In the gas phase, IR emission spectra have been studied for neutral PAHs and PAH cations (Cook et al 1998;Kim et al 2001;Kim & Saykally 2002). Absorption gas-phase spectra of PAH cations have been studied using IR multiphoton dissociation (IRMPD; Oomens et al 2001Oomens et al , 2006 or messenger atom photodissociation spectroscopy (Piest et al 1999;Duncan 2003;Douberly et al 2008;Ricks et al 2009).…”

Section: Introductionmentioning

confidence: 99%

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Jiang

Castellanos

Bouwman

et al. 2017

ApJ

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We present the first infrared (IR) gas-phase spectrum of a large and astronomically relevant polycyclic aromatic hydrocarbon (PAH) cation (C 42 H 18 + , HBC + ) and its dication (C 42 H 18 + 2 , HBC 2+ ). The spectra are recorded via infrared multiphoton dissociation (IRMPD) spectroscopy of ions stored in a quadrupole ion trap, using the intense IR radiation of a free electron laser in the 530-1800 cm −1 (5.6-18.9 μm) range. HBC + shows main intense absorption peaks at 762 (13.12), 1060 (9.43), 1192 (8.39), 1280 (7.81), 1379 (7.25), and 1530 (6.54) cm −1 (μm), in good agreement with density functional theory calculations after scaling to take the anharmonicities effect into account. HBC 2+ has its main absorption peaks at 660 (15.15), 766 (13.05), 1054 (9.49), 1176 (8.50), 1290 (7.75), 1370 (7.30) and 1530 (6.54) cm −1 (μm). Given the similarity in the cationic and dicationic spectra, we have not identified an obvious diagnostic signature to the presence of multiply charged PAHs in space. While experimental issues associated with the IRMPD technique preclude a detailed comparison with interstellar spectra, we do note that the strong bands of HBC + and HBC 2+ at ∼6.5, 7.7, 8.4, and 13.1 μm coincide with prominent aromatic infrared bands (AIBs). HBC has only trio CH groups, and the out-of-plane CH bending mode of both HBC cations is measured at 13.1 μm, squarely in the range predicted by theory and as previously found in studies of small (substituted) PAHs. This study therefore supports the use of AIBs observed in the 11-14 μm range as a diagnostic tool for the edge topology of large PAHs in space.

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Section: Introductionmentioning

confidence: 99%

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Jiang

Castellanos

Bouwman

et al. 2017

ApJ

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“…Léger & Puget (1984) and Allamandola et al (1985) proposed polycyclic aromatic hydrocarbons (PAHs) as the carriers of these bands. Despite two decades of experimental and theoretical efforts, the match (band positions and intensities) between data only available on small species and observations still remains elusive, as illustrated recently by the work of Peeters et al (2002) and Kim & Saykally (2002). Given their important role in the ISM (e.g., gas heating, Habart et al 2001; UV extinction, Joblin et al 1992), it is necessary to find other ways to constrain the properties of interstellar PAHs.…”

Section: Introductionmentioning

confidence: 99%

The long-wavelength emission of interstellar PAHs: characterizing the spinning dust contribution

Ysard¹,

Verstraete²

2010

A&A

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Context. The emission of cold dust grains at long wavelengths will soon be observed by the Planck and Herschel satellites and will provide new constraints on the nature of interstellar dust. In particular, the microwave galactic anomalous foreground detected between 10 to 90 GHz, proposed as coming from small spinning grains (PAHs), should help to define these species better. Moreover, understanding the fluctuations of the anomalous foreground quantitatively over the sky is crucial for CMB studies. Aims. We focus on the long-wavelength emission of interstellar PAHs in their vibrational and rotational transitions. We present here the first model that coherently describes the PAH emission from the near-IR to microwave range. Methods. We take quantum effects into account to describe the rotation of PAHs and compare our results to current models of spinning dust to assess the validity of the classical treatment used. Between absorptions of stellar photons, we followed the rovibrational radiative cascade of PAHs. We used the exact-statistical method of Draine & Li to derive the distribution of PAH internal energy and followed a quantum approach for the rotational excitation induced by vibrational (IR) transitions. We also examined the influence of the vibrational relaxation scheme and of the low-energy cross-section on the PAH emission. We study the emissivity of spinning PAHs in a variety of physical conditions (radiation field intensity and gas density), search for specific signatures in this emission that can be looked for observationally, and discuss how the anomalous foreground may constrain the PAH size distribution. Results. Simultaneously predicting the vibrational and rotational emission of PAHs, our model can explain the observed emission of the Perseus molecular cloud from the IR to the microwave range with plausible PAH properties. We show that for λ ≥ 3 mm the PAH vibrational emission no longer scales with the radiation field intensity (G 0 ), unlike the mid-IR part of the spectrum (which scales with G 0 ). This emission represents less than 10% of the total dust emission at 100 GHz. Similarly, we find the broadband emissivity of spinning PAHs per carbon atom to be rather constant for G 0 ≤ 100 and for proton densities n H < 100 cm −3 . In the diffuse ISM, photon exchange and gas-grain interactions play comparable roles in exciting the rotation of PAHs, and the emissivity of spinning PAHs is dominated by the contribution of small species (bearing less than 100 C atoms). We show that the classical description of rotation used in previous works is a good approximation and that unknowns in the vibrational relaxation scheme and low-energy cross-section affect the PAH rotational emissivity around 30 GHz by less than 15%. Conclusions. The contrasted behaviour of the PAH vibrational and rotational emissivities with G 0 provides a clear prediction that can be tested against observations of anomalous and dust mid-IR emissions: this is the subject of a companion paper. Comparison of these emissions complem...

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“…IR emission spectra were reported for the pyrene cation (C 16 H + 10 ) and several dehydrogenated derivatives (Kim & Saykally 2002;Kim et al 2001). The pyrene cation spectrum showed a good correspondence with matrix isolation and IRMPD spectra of the pyrene cation.…”

Section: Ir Emission Spectroscopymentioning

confidence: 83%

Laboratory infrared spectroscopy of PAHs

Oomens

2011

EAS Publications Series

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Single‐Photon Infrared Emission Spectroscopy of Gaseous Polycyclic Aromatic Hydrocarbon Cations: A Direct Test for Proposed Carriers of the Unidentified Infrared Emission Bands

Cited by 54 publications

References 56 publications

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

The long-wavelength emission of interstellar PAHs: characterizing the spinning dust contribution

Laboratory infrared spectroscopy of PAHs

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Single‐Photon Infrared Emission Spectroscopy of Gaseous Polycyclic Aromatic Hydrocarbon Cations: A Direct Test for Proposed Carriers of the Unidentified Infrared Emission Bands

Cited by 54 publications

References 56 publications

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC+ and HBC2+

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC+ and HBC2+

The long-wavelength emission of interstellar PAHs: characterizing the spinning dust contribution

Laboratory infrared spectroscopy of PAHs

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Resources

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Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺

Infrared Spectra of Hexa-peri-hexabenzocoronene Cations: HBC⁺ and HBC²⁺