Testing the “strong” PAHs hypothesis

Abstract

Abstract. In Paper I (Malloci et al. 2003) we proved the profile invariance of the first permitted electronic transition of the typical Polycyclic Aromatic Hydrocarbon cation C 32 H 14 + as a first necessary check for the "strong" PAHs hypothesis. In this paper we derive a quantitative relation between the intensities of the former band, which ought to be observable in absorption in the visible range, and those of the far-IR bands, which are predicted by the PAH model to be simultaneously present in emission. Contrary to the mid-IR bands, collectively known as "Unidentified Infrared Bands" (UIBs), which do not discriminate specific molecules, the far IR, skeletal bands can be expected to be a fingerprint of each single species. This fact provides a number of independent constraints which must be simultaneously fulfilled for a successful PAH identification. Our approach thus offers a powerful criterion for the identification of specific PAHs, both in the presently available ISO data and in those of the forthcoming SIRTF and Herschel missions. As an interesting by-product, we quantitatively evaluate the impact of isotopic substitutions ( 13 C→ 12 C and D →H) on the resulting infrared emission bands.

Computational IR Spectroscopy for PAHs: from the early years to the present status

“…A non exhaustive list of examples is given below focusing on the physical effects, independently of the size of the PAH carriers. Among the effects considered one may cite: -multiple ionizations Bauschlicher & Bakes 2000;Bakes et al 2001;) -negative ions (Langhoff 1996;Bakes et al 2001;Bauschlicher et al 2008) -dehydrogenated PAHs (Pauzat et al 1995(Pauzat et al , 1997 (Langhoff et al 1998;Pauzat et al 1999;Hudgins et al 2005) -irregular structures with 4/5/7 membered rings (Bauschlicher et al 1999;Pauzat & Ellinger 2002) -deuterated PAHs Mulas et al 2003;Peeters et al 2004) -metal complexation (Fe, Mg, Si) Cassam 2002;Hudgins et al 2005;Bauschlicher & Ricca 2009;Joalland et al 2010;Simon & Joblin 2007 . .…”

Identifyingspecificinterstellar polycyclic aromatic hydrocarbons

“…In previous papers [13,14], we demonstrated the use of a Monte-Carlo model to simulate the detailed photophysics of a specific PAH, namely neutral and cationic ovalene (C 32 H 14 ), embedded in a given radiation field (RF). A similar approach for neutral coronene (C 24 H 12 ) were previously used by [12].…”

“…Figure 1 shows the comparison between ab initio [17] and laboratory [16] photo-absorption cross sections for the anthracene and ovalene molecules, and the impact of their differences on the estimated absorption rates in three vis/UV RFs. We considered the average ISRF 5 Kpc away from the galactic centre and the RFs of the photodissociation region of the Red Rectangle reflection nebula and of the planetary nebula IRAS 21282+5050 [13,14]. For both molecules the agreement between synthetic and laboratory spectra is reasonably good, with typical differences of ∼0.3 eV between calculated and measured energies of the absorption bands.…”

“…On the other hand, each specific PAH ought to show a unique spectral fingerprint in the far infrared region, which contains the low-frequency vibrational modes associated with collective oscillations of the whole carbon skeleton of the molecule [9,11,12,13]. Moreover, far-IR bands, in the PAH hypothesis, are expected to be emitted preferentially when the excitation energy of the molecule is relatively low [12]; this would result in slower internal vibrational redistribution of energy and thus smaller lifetime broadening, implying that their rotational envelopes, or even their fully resolved rotational structure, might be still discernible, as hinted by the gas phase laboratory measurements of [11].…”

“…The far-IR spectrum emitted by a given interstellar PAH will depend essentiallly just on its molecular properties and on the exciting radiation field, and can be modelled in detail if all these "ingredients" are known [12,13,14]. The stumbling block in performing such a calculation in a systematic way for a large sample of specific PAHs and radiation fields is indeed in the lack of said "ingredients": there are precious few species for which both the complete vibrational analysis and the photoabsorption spectrum up to the Lyman limit are known, to date.…”