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New Technique to Probe Chemical Dynamics of Combustion

4 January 2006

Sliced ion imaging

Chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, working with colleagues at Stony Brook University, have developed a new experimental technique to measure the flow of energy inside a molecule in the process of breaking apart.

The experiments provide a test of theories used in computer models of combustion, which are used, for instance, by combustion engineers to design more fuel-efficient and less polluting machines.

The chemists used an experimental technique called sliced ion imaging to analyze the energy flow during the fragmentation of ketene, a small molecule that acts as a stand-in for more complicated fuels.

When excited with a laser, ketene breaks apart into carbon monoxide and methylene fragments, pushing away from each other with a speed that depends on the rotational and vibrational energies of the two fragment molecules.

Some of the carbon monoxide fragments are then ionized (an electron is removed) by a second laser, and the ions are accelerated toward a detector screen, where each ion shows up as a flash of light, observed with a video camera. Depending on how fast and in what direction the carbon monoxide fragments were traveling when they were born, the ions will hit a different spot on the ion-imaging detector.

The slicing is a key new feature of the method, which makes the image sharper and more accurate.

By analyzing the images obtained by accumulating the signals from about half a million molecules, the Brookhaven scientists can determine how energy is shared between the two fragments. They have found agreement with the predictions of a chemical rate theory called variational transition state theory, refuting some earlier work that had raised questions about this theory.

With a trustworthy computer model of combustion, engineers can design improved engines or fuel blends without needing to build and test so many different versions. The database of chemical reactions needed for such a computer program is too big to be filled in strictly with measured numbers, and we have to rely on chemical rate theory to calculate much of this database. Our experiments are important for showing how far we should trust these calculations.
—Gregory Hall, Brookhaven principal researcher

The research is published in today’s online edition of the Journal of Chemical Physics.

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