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Medium action

by Barbara Jewett

University of Colorado researchers study the chemistry of the interstellar medium with the help of NCSA’s Abe.

Space is not empty.

In fact, says Zhibo Yang, there’s an area of the universe called the interstellar medium (ISM) that is filled with molecules, atoms, ions, and grains of dust. And these things are conducting some very interesting chemical activity.

Ion-molecule reactions are dominant among these reactions. Yang, a post-doctoral researcher at the University of Colorado, Boulder, and his colleagues are studying reactions that involve negative ions with the help of the Abe supercomputer at NCSA.

“Positive ions have been relatively well studied,” says Yang. “But negative ions have only recently been detected in the ISM.”

More than 160 molecular species have been detected in the ISM. And evidence is mounting, he says, in support of the existence of large organic species such as polycyclic aromatic hydrocarbons (PAH). But little is known about the chemical origins of these species. A complete understanding of the evolution and interconversion of species in the ISM will require detailed data on the chemical and physical processes occurring throughout space.

Yang’s team is looking at the reaction between ions and neutrals, measuring the rate constants of these reactions, and the mass to charge ratio of the negative ions using mass spectrometry. But mass spectrometry yields no information regarding the ion structure. To get information about the structures and energies and to understand their reactivity requires calculations. Very large calculations.

“In principle, some small calculations can be run on our PCs, but for the large calculations we can not affordably carry them out on our PCs. Also, our PCs do not have enough memory, they crash with the large calculations,” explains Yang. “The typical calculation will last for 12 hours on our PC but we can do the same calculation on Abe in about two to three hours.”

So far, the group has completed both experimental and theoretical studies of reactions between atomic species (i.e., hydrogen, nitrogen, and oxygen) and anions (i.e., carbon chain anions and some typical molecular anions). Five oral presentations have been made at international and domestic conferences and six papers were published in 2010, in the Journal of the American Chemical Society, the Astrophysical Journal, and in Physical Chemistry Chemical Physics.

While the team does some experimental work, Yang says the low temperatures of the ISM and the diverse pressures usually cannot be replicated in a laboratory setting. In a computer simulation, however, researchers can easily set the temperature and pressure. The simulations verify experimental results, and the team’s computational work can also help predict the possibility of undiscovered species. And they are providing some fundamental science, general rules to help scientists understand other related reactions.

There are other scientists who are doing modeling, explains Yang, and they want to use the reactions with rate constants in their models. Rate constants are important for astrochemical modeling to establish the reaction networks among species in the ISM. If they can determine when reactions occurred, eventually scientists can predict the evolution of the universe.

While the team’s current research may seem esoteric, he notes, other scientists use the results for their research projects that are likely to draw more public attention. But the team’s research successes are helping their careers. Yang says he’s had several interviews in his quest to secure an assistant professorship and one PhD student just finished his thesis defense and has a position at Harvard for postdoctoral research. When settled at a new university, Yang hopes to do studies in other areas that will attract more public attention, such as biomolecules, catalytic reactions, and renewable energy. With the help of NCSA, of course.

This project was funded by the National Science Foundation and NASA.

Team members
Veronica M. Bierbaum
Theodore P. Snow
Zhibo Yang
Oscar Martinez Jr.
Nicholas J. Demarais
John M. Garver
Callie A. Cole
Charles Nichols
Marshall Carpenter
Denver Hager

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