Charged polycyclic aromatic
hydrocarbon clusters and the galactic red emission
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In the March 27th, 2007
edition of the Proceedings
of the National Academy of Sciences, a team of scientists from the Space Science Division at
NASA Ames Research Center, The University of California at Berkeley, and
the University of Maryland report that the carrier of a mysterious red glow
that is present throughout the Milky Way and other galaxies could be due to
very unusual, nano-sized, carbon and hydrogen rich particles made of polycyclic
aromatic hydrocarbons (PAHs).
Using pioneering computational methods to test their ideas, this
interdisciplinary team of experimental and theoretical chemists and astronomers
propose that a very unusual form of charged PAH cluster is responsible for the
red glow. These nano-sized
clusters do not exist on Earth because they are very reactive and short lived.
They can only be prepared under very special conditions in the laboratory.
However, they could thrive under the harsh conditions in space. Indeed their preparation requires the harsh environment of space.
This work was supported by National Aeronautics and Space Administration Long Term Space Astrophysics and Astrobiology (NAI) Programs Grants NNGO4GB94G, 399-20-40,and 344-53-92; and the director of the Office of Energy Research, Office of Basic Energy Sciences,Chemical Sciences Division of the U.S. Department of Energy under Contract DE-AC03-76SF00098 (toY.M.R.andM.H.-G.).
More than 30 years ago astronomers
discovered a surprising deep red glow coming from an odd object in the
sky. This strange red object had
the shape of a rectangle in earlier photographs and came to be known as the Red Rectangle. The beautiful Hubble Space Telescope
image shows the Red Rectangle in all of its wonder. This red glow, renamed the Extended Red Emission, or ERE, is
now known to be widespread throughout our Galaxy (The Milky Way) and has been
observed in other galaxies, showing that the carrier is a common and important
cosmic ingredient. Over the years
astronomers have put tight constraints on the properties that the glowing
material must have, eliminating all of the possible carriers suggested up to
now. They also found that the
highly luminescent material must be extremely tough. It appears to require very harsh ultraviolet radiation to
become luminescent, a radiation field so strong that most known polyatomic
interstellar molecules would be destroyed.
During this same time, astronomers realized that large, chicken-wire shaped, carbon-rich molecules called polycyclic aromatic hydrocarbons (PAHs) were present in nearly all of the places that emitted the red glow. PAHs are robust molecules that can withstand the rigors of deep space in all but the most extreme environments. This coincidence of PAHs with the ERE is strong circumstantial evidence for a connection. However, while PAHs can be very luminescent, different PAHs and PAH clusters glow at all colors of the rainbow, not just in the red, so simple PAHs cannot be the long sought ERE carriers. 'We have been studying PAHs in the laboratory at NASA Ames for a long time, and although I had results which strongly supported the idea that PAHs had something to do with the ERE, the experimental results made it clear that if PAHs were involved, they were present in some as-yet exotic form' said Murthy Gudipati of the University of Maryland, who joined JPL very recently. Indeed, the team proposes that the ERE is carried by nano-sized charged PAH clusters with two properties that are, in a sense, opposite to those in all stable materials on earth. They are both charged and yet have a closed-shell electron configuration needed for the material to be stable, at the same time. That is, the electron distribution in these clusters is the same as in garden variety, stable, unreactive molecules on Earth. These types of highly reactive species are simply not readily accessible for laboratory study, but need very special conditions,' continued Gudipati.
Tim Lee, Chief of the Space Science Division at NASA Ames and Martin Head-Gordon, Professor of Chemistry, at the University of California in Berkeley, world renowned theoretical chemists, realized that recent advances in theoretical techniques and computer hardware made it possible to tackle this problem computationally. 'After trying several methods we developed for large systems such as these, we ran into problems modeling the transfer of charge within these nano-clusters. Since it is this charge transfer from one site in the PAH
cluster to another site that would produce the glow, we had to develop a totally new approach,' said Lee. After several different failed attempts, the team came up with a novel approach. 'We modified a standard wave function approach by inserting a resolution of the identity. This allowed us to treat the complete system while still employing a method that is accurate for charge transfer states,' said Head-Gordon. 'Once we showed this new approach could handle this problem, I was able to simulate the detailed emission process on molecular systems much larger than any that had been done before,' said Young Min Rhee, a post-doctoral student with Professor Martin Head-Gordon and the lead author. 'Our simulation shows that this type of charged PAH cluster has an emission entirely consistent with the ERE' said Lou Allamandola, Head of the NASA Ames Astrochemistry Laboratory, who stated further 'that these charged nano-clusters are also
consistent with the need for a harsh radiation field, and a two step formation mechanism, one of the observational constraints placed on the source of the ERE.'
This work has important implications in many other areas as well, including combustion processes and nano-materials. Regarding combustion, the formation of polluting soot particles by, say, diesel and jet engines, is important from an environmental and health perspective. Yet, how the soot particles are formed in flames is not well understood, nor has the carrier of a puzzling reddish luminescence which is associated with these flames been identified. Interestingly, it has been suggested that self-forming PAH clusters may be the key first step, but as mentioned above, normal PAHs or their clusters alone cannot produce a red glow. Additionally, there is fleeting evidence for the presence of closed-shell charged PAH ions in flames. Thus, the closed-shell PAH cluster ions proposed here may not only hold the key to understanding the ERE throughout the cosmos, but may also help explain the nucleation and soot forming process in polluting flames. Concerning advanced materials, 'The self-assembly of PAH molecules is important because a well ordered PAH stack (an ordered, charged PAH supercluster) could well form the backbone of novel nano-materials with exceptionally high charge mobility. As discussed for the first time in this paper, charged PAH clusters have dramatically different characteristics compared with simple neutral stacked PAHs' concluded Head-Gordon.
Authors:
Y.M. Rhee (University of California, Berkeley)
T.J. Lee (NASA Ames Research Center)
M.S. Gudipati (Jet Propulsion Laboratory)
L.J. Allamandola (NASA Ames Research Center)
M. Head-Gordon (University of California, Berkeley)
For more information please see:
http://www.pnas.org/cgi/content/full/104/13/5253
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