Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

doi:10.1073/pnas.0609396104

Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

^†Department of Chemistry, University of California, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720;
^‡Astrochemistry Laboratory, National Aeronautics and Space Administration/Ames Research Center, MS 245-6, Moffett Field, CA 94035; and
^§Institute for Physical Sciences and Technology, University of Maryland, College Park, MD 20742

Edited by Louis E. Brus, Columbia University, New York, NY, and approved February 1, 2007 (received for review October 23, 2006)

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 ≈540 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.

Footnotes

^¶To whom correspondence may be addressed. E-mail: lallamandola{at}mail.arc.nasa.gov or mhg{at}cchem.berkeley.edu
Author contributions: T.J.L., L.J.A., and M.H.-G. designed research; Y.M.R. and M.S.G. performed research; Y.M.R. analyzed data; and Y.M.R., T.J.L., L.J.A., and M.H.-G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0609396104/DC1.
Abbreviations:

ERE,

extended red emission;

PAH,

polycyclic aromatic hydrocarbon;

MP2,

second-order Møller–Plesset perturbation correction;

RI,

resolution of the identity;

CIS(D),

configuration interaction singles with perturbative doubles corrections;

TDDFT,

time-dependent density functional theory;

CCSD(T),

coupled-cluster singles and doubles with perturbative triples correction.