Interstellar Chemistry:  A Strategy for Detecting Polycyclic Aromatic Hydrocarbons in Space

Lovas, Francis J.; McMahon, Robert J.; Grabow, Jens‐Uwe; Schnell, Melanie; Mack, James; Scott, Lawrence T.; Kuczkowski, Robert L.

doi:10.1021/ja0426239

Cited by 172 publications

(145 citation statements)

References 32 publications

(51 reference statements)

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“…We thus assume that the rotational constant B is the same in all vibrational levels, and within the framework of a rigid rotor model, we neglect the centrifugal distortion terms in the energy equation that are usually small for large molecules (Herzberg 1968ab;Lovas et al 2005). …”

Section: The Rigid Rotor Modelmentioning

confidence: 99%

“…For instance, a PAH having lost one H atom has μ ∼ 0.8 to 1 D, and a PAH cation where a C atom has been substituted by N would also have μ ∼ 0.1 to 1.5 D depending on its size (Pino, private communication). Moreover, it has been proposed that nonplanar PAHs containing pentagonal rings may exist in the ISM (see Moutou et al 2000, and references therein): such species are known to have large dipole moments as recently measured on coranulene, C 20 H 10 , μ = 2 D (Lovas et al 2005). In this work, an MC (G 0 = 10 −2 ), b) the diffuse ISM (G 0 = 1) and c) the Orion Bar (the radiation field is the sum of the CMB, the ISRF and a blackbody at 37 000 K corresponding to G 0 = 14 000).…”

Section: Electric Dipole Momentmentioning

confidence: 99%

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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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Section: The Rigid Rotor Modelmentioning

confidence: 99%

Section: Electric Dipole Momentmentioning

confidence: 99%

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

Ysard¹,

Verstraete²

2010

A&A

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“…5 Owing to its large permanent dipole moment 6 of around 2.1 D, corannulene has been the target of several laboratory and observational microwave studies. 7 As compared to planar regular PAHs, corannulene offers an opportunity to detect an individual PAH molecule in space based on its microwave spectrum. Density functional theory (DFT) methods have been extensively applied to planar PAH systems 8 and they have proven to yield calculated harmonic IR frequencies which are in good agreement with experimental IR vibrational spectra, both for neutral and radical cation forms 9,10 as well as for positively charged PAH derivatives.…”

Section: Introductionmentioning

confidence: 99%

Infrared spectroscopy of ionized corannulene in the gas phase

Galué

Rice

Steill

2011

The Journal of Chemical Physics

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The gas-phase infrared spectra of radical cationic and protonated corannulene were recorded by infrared multiple-photon dissociation (IRMPD) spectroscopy using the IR free electron laser for infrared experiments. Electrospray ionization was used to generate protonated corannulene and an IRMPD spectrum was recorded in a Fourier-transform ion cyclotron resonance mass spectrometer monitoring H-loss as a function of IR frequency. The radical cation was produced by 193-nm UV photoionization of the vapor of corannulene in a 3D quadrupole trap and IR irradiation produces H, H 2 , and C 2 H x losses. Summing the spectral response of the three fragmentation channels yields the IRMPD spectrum of the radical cation. The spectra were analyzed with the aid of quantum-chemical calculations carried out at various levels of theory. The good agreement of theoretical and experimental spectra for protonated corannulene indicates that protonation occurs on one of the peripheral C-atoms, forming an sp 3 hybridized carbon. The spectrum of the radical cation was examined taking into account distortions of the C 5v geometry induced by the Jahn-Teller effect as a consequence of the degenerate 2 E 1 ground electronic state. As indicated by the calculations, the five equivalent C s minima are separated by marginal barriers, giving rise to a dynamically distorted system. Although in general the character of the various computed vibrational bands appears to be in order, only a qualitative match to the experimental spectrum is found. Along with a general redshift of the calculated frequencies, the IR intensities of modes in the 1000-1250 cm −1 region show the largest discrepancy with the harmonic predictions. In addition to CH "in-plane" bending vibrations, these modes also exhibit substantial deformation of the pentagonal inner ring, which may relate directly to the vibronic interaction in the radical cation.

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“…C 24 H 12 is a highly symmetric peri-condensed PAH (D 6h ), while C 20 H 10 (C 5v ) presents some unique properties for astronomical detection because of its dipole moment of 2.1D (Lovas et al 2005). Structurally, C 20 H 10 is the smallest PAH adopting a bowl shape, given by its central five-membered ring and being a third of C 60 .…”

Section: Introductionmentioning

confidence: 99%

Gas-phase Electronic Spectra of Coronene and Corannulene Cations

Hardy

Rice

Maier

2017

ApJ

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Gas-phase electronic spectra of the coronene ( + C H 24 12 ) and corannulene ( + C H 20 10 ) cations complexed with helium have been recorded in a quadrupole ion trap at 5 K by photodissociation. The electronic spectrum of + C H 20 10 with two helium atoms was also measured to estimate the perturbation. This method is sufficient for an astronomical comparison because the shift due to the weakly bound helium is on the order of 0.2 Å.

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Interstellar Chemistry: A Strategy for Detecting Polycyclic Aromatic Hydrocarbons in Space

Cited by 172 publications

References 32 publications

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

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

Infrared spectroscopy of ionized corannulene in the gas phase

Gas-phase Electronic Spectra of Coronene and Corannulene Cations

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