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Combustion and Interstellar Chemistry

The David Nesbitt group is investigating the chemistry of combustion on Earth and chemistry in outer space. The two processes lead to similar products, but on Earth the formation of ash occurs at much higher temperatures and pressure than exist in the interstellar medium (ISM) where similar chemicals are found. One important goal is to discover how molecules present in ash are formed in outer space. A second goal is to unravel the chemical reactions that take place when flames turn ordinary matter into ash. There are hints that “cosmo” chemistry and combustion chemistry may have some reaction pathways in common.
This idea comes from information gathered by radio telescopes. These instruments have found evidence of molecules made of long chains of carbon atoms, some of which consist of dozens of six-carbon rings in the ISM. In other words, interstellar clouds contain pieces of tar. The Nesbitt group wants to know how these molecules were formed in space. One clue has been the identification of ethynyl radical in the Orion Nebula and other interstellar clouds.

Nesbitt believes that ethynyl radical is the key to understanding the transformation of simple molecules into ash. Once it is produced, ethynyl radical rapidly reacts with other carbon-containing molecules, producing increasingly complex molecules. The end result is the long-chained molecules of carbon and hydrogen that make up soot and ash.

Nesbitt’s group is currently using high-resolution laser spectroscopy to study the formation of hydroxyformyl or HOCO radical, ethynyl radical, vinyl radical, cyclopentadienyl radical, and other precursors of  ash.  By obtaining “quantum fingerprints” of these molecules, the group is coming closer to understanding combustion on Earth and chemistry in the cosmos.

The researchers obtain their spectra after first using a supersonic expansion to cool hot molecules (~1000 K) down to 10–20 K. The low temperature is critical for their investigations. At very low temperatures, the molecules exist in sufficiently few quantum states that their spectra are possible to analyze. The supersonic-expansion technique is one of the best in the world for conducting infrared spectroscopy (IR) of highly reactive molecules.

Molecules currently under study in the laboratory include the highly reactive phenyl radical, which is implicated in the chemistry of the ISM, and acetylene (C2H2). Removing one of the hydrogen atoms from acetylene produces ethynyl radical that, in turn, reacts with acetylene to make diacetylene. These molecules keep adding together to make huge, very long molecules. The group hypothesizes that this process may be common to combustion processes that end up forming ash.

The group expects their work on phenyl and other radicals in the laboratory to play in an important role in the future of IR astrophysics. Many symmetric molecules cannot be seen with radio telescopes because they lack a dipole moment. However, such molecules will be visible to IR telescopes expected to come online during the next decade. The Nesbitt group’s lab work will help astronomers predict the location of the IR spectral lines of phenyl radical and other important molecules in combustion and interstellar chemistry.

The Nesbitt group also uses laser spectroscopy to study important molecules and ions, such as ammonium ion (NH4+) and its deuterium-containing isoptomers as well as H5+, that exist in space and whose spectra are visible in the IR. The latter, H5+, is one of the most stable ions in the universe, but thus far no one has been able to obtain a high-resolution spectrum of it. The Nesbitt group is determined to be the first to accomplish this goal.