Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

Abstract

The species responsible for the broad extended red emission (ERE), discovered in 1975 and now known to be widespread throughout the Galaxy, still is unidentified. Spanning the range from Ϸ540 to 900 nm, the ERE is a photoluminescent process associated with a wide variety of different interstellar environments. Over the years, a number of plausible candidates have been suggested, but subsequent observations ruled them out. The objects that present the ERE also emit the infrared features attributed to free polycyclic aromatic hydrocarbon (PAH) molecules, suggesting that closely related materials are plausible ERE carriers. Here, we show that the peculiar spectra and unique properties of closed-shell cationic PAH dimers satisfy the existing observational constraints and suggest that emission from mixtures of charged PAH clusters accounts for much of the ERE. This work provides a view into the structures, stabilities, abundances, and ionization balance of PAH-related species in the emission zones, which, in turn, reflects physical conditions in the emission zones and sheds fundamental light on the nanoscale processes involved in carbon-particle nucleation and growth and carbonaceous dust evolution in the interstellar medium.dimer ͉ excited state ͉ interstellar medium ͉ nanoparticles T he extended red emission (ERE) was discovered in 1975 by Cohen et al. (1) in the peculiar object known as the Red Rectangle. Subsequent observations have shown that it is a widespread interstellar photoluminescent phenomenon. Witt and coworkers established that the ERE is associated with reflection nebulae (2, 3), carbon-rich planetary nebulae (4), and the interstellar medium of the Galaxy (5), whereas Darbon, Perrin, Sivan, and coworkers have detected the ERE in galactic HII regions and other galaxies (6-8). A detailed review can be found elsewhere (9).The ERE is a broad emission band starting near 540 nm that extends into the near infrared (IR). For most objects, the peak wavelength varies between 600 and 700 nm (3), although in the HII region of the Orion nebula, the band seems to peak near 800 nm (8). These variations are thought to arise from modest changes in chemical make-up and differing radiation fields and not from fundamental changes in the nature of the carrier. In some cases, structure appears superposed on the band (10, 11). Over the years a number of possible explanations have been suggested, including photoluminescence from species such as neutral polycyclic aromatic hydrocarbon (PAH) molecules (12), organic residues (13), neutral PAH clusters (14), hydrogenated amorphous carbon particles (15, 16), and silicon nanoparticles (17). Although a carbon-rich carrier was implicated by cosmic abundance constraints, all of these materials have been found wanting in light of subsequent observations. The case for each of these suggestions and the observational difficulties they encounter are summarized nicely by Witt et al. (18). Recently, observational constraints have been determined that are especially important in revealing t...

Extended Red Emission and the evolution of carbonaceous nanograins in NGC 7023

“…Finally, laboratory spectroscopic studies on [PAH 2 ] + are needed to progress in this identification. In particular, closed-shell cation dimers as proposed by Rhee et al (2007) deserve additional studies.…”

“…In a recent theoretical work (Rhee et al 2007), it was shown that charged PAH dimers ([PAH 2 ] + ), more specifically the subclass made of closed-shell species, can fluoresce in the ERE range with a quantum yield that is consistent with this emission. The overlays of Figs.…”

Initiating molecular growth in the interstellar medium via dimeric complexes of observed ions and molecules

“…Recently, two fullerene molecules (C60 and C70) have been identified in a carbon rich planetary nebulae (Cami et al 2010). It is proposed that the identification of individual PAH molecules could help to resolve some of the mysteries associated with the diffuse interstellar bands (DIBs; Salama et al 1999;Snow 2001) and the galactic extended red emission (ERE; Rhee et al 2007). Similar to PAHs, spectroscopic signatures of nitrogen containing polycyclic aromatic hydrocarbons (PANH) are prevalent in various cosmological environments (Hudgins et al 2005).…”

“…However, the cationic complexes are bound by both electrostatic and covalent interactions. Closed and open shell PAH dimers are estimated to be bound by energies of about 20 kcal mol −1 , and absorb in the visible and sometimes in the infrared (Rhee et al 2007;Weisman et al 2005). The cationic complexes of small olefins, especially the nitrogen containing ones, have not been explored in any detail.…”

PAHs in Astronomy - A Review

“…The carriers of the ERE are widespread throughout the diffuse interstellar medium of the Milky Way Galaxy and other galaxies (Rhee et al 2007, Berné et al 2008, Witt et al 2008.…”