Brookhaven Chemical Physics

While the field of physics generally strives to find compact and universal explanations for how the components of our universe interact, chemistry is traditionally considered an applied field of physics, celebrating and exploring the diversity of matter and its properties. The rules of physics that govern most of chemistry were worked out in the early 20th century and have since then proved satisfactory. The consequences of these rules are complex and continue to be explored.

The field of chemical physics aims to understand, predict, or control the behavior of matter in an expanding range of chemical systems—systems simple enough that accurate, trustworthy theoretical calculations of their behavior are feasible and, as a result, that any differences between experimental results and theoretical predictions could indicate something unexpected is happening. In that light, a chemical physicist will typically focus on problems that a physicist might consider plagued by unnecessary detail, and that a chemist might distain as too elementary to be of practical importance.

One active field of chemistry that is increasingly becoming better understood is catalysis. A catalyst is a substance that accelerates or directs a chemical reaction to occur with lower energy input or fewer byproducts, without itself being consumed in the reaction. Catalysts generally work by temporarily holding reactant molecules in a particular orientation or energy state during a reaction. The catalytic removal of smog-forming hydrocarbons, nitrogen oxides, and toxic carbon monoxide from car exhaust is a familiar example of catalysis that makes modern automobiles dramatically cleaner than their predecessors.

The tools needed to observe, manipulate and predict these molecular-scale properties have begun to change catalysis from an empirical science of trial and error to a scientific field grounded in testable theory. Atomic-scale imaging microscopy, laser, and x-ray tools can watch molecules bind, migrate, and break away from surfaces, allowing scientists to investigate the connection between a surface’s structure and its chemical behavior. The Center for Functional Nanomaterials, soon to begin construction at Brookhaven, is bringing even more powerful new tools into play in the development of catalysis science at the Lab.

Another focus of chemical physics research at Brookhaven is on laser spectroscopy and chemical dynamics. The aim of this work is to understand chemical reactions at the level of individual molecular collisions. Lasers can impart specific energies to molecules in a dilute gas sample, and can also detect their fate during and after reactions or collisions. By measuring how the speed of a reaction or the energy of the reaction products depends on the energy of the participating molecules, the theories used to predict chemical reactivity can be tested and improved. The computer models used in designing improved engines, turbines, and other fuel-burning devices rely on such theories to calculate the efficiency and emissions of new designs without the need to build and test them.

Brookhaven chemists have developed the Laser-Electron Accelerator Facility (LEAF), which uses laser light to produce short bunches of electrons that are accelerated to high energies. The electrons penetrate liquid samples, creating a shower of several different, highly reactive ions and molecules, as well as free electrons. Chemists are interested in the properties of these unstable species and how they react—which type of reactions occur, how fast they occur, and what the reaction products are.

One problem currently under investigation at LEAF is electron transfer through long chain molecules that act as molecular “wires.” In a solution, a free electron, produced by energetic LEAF electrons, can be picked up by a molecular wire. If the wire is a good conductor, the electron will travel along its length to a probe molecule, which can trap the electron. The time it takes for this to occur allows the experimenters to learn more about what makes a good molecular conductor. Molecular wires may find use in future generations of solar energy conversion devices or in microelectronics.

The Nobel-prize winning research done by Brookhaven’s Ray Davis is rooted in Brookhaven’s Chemistry Department. Davis won the 2002 Nobel Prize in Physics for his work begun in the 1960’s with a large radiochemical experiment that detected solar neutrinos—tiny neutral particles produced by nuclear fusion reactions in the Sun. To detect neutrinos, the experiment employed a huge tank of chlorinated solvent, located in a South Dakota gold mine to shield it from everyday background radiation. The numerous but nearly undetectable neutrinos occasionally triggered the conversion of a chlorine nucleus into a radioactive form of argon, which was chemically separated from tons of chlorinated solvent and measured one atom at a time by counting its radioactive decays.

Davis found fewer neutrinos arriving at the earth than theories of the sun’s energy production predicted, and posed what became known as the Solar Neutrino Problem. Subsequent work by international collaborations of neutrino scientists, including members of the Brookhaven Chemistry Department, has confirmed Davis’s results. They concluded that the reason for the discrepancy is an unexpected transformation that occurs among neutrinos. There are three kinds of neutrinos, but, primarily, only one kind is produced by the Sun. Before those neutrinos reach Earth, many change into one of the other types. Because Davis’ experiment could detect only one variety, it recorded a lower neutrino count than expected.

Neutrino physics is now a frontier topic in the search for the next generation of particle physics theories, and chemists at Brookhaven are working on designing new detectors for future neutrino experiments.

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Last Modified: January 4, 2006