ODU PHYSICISTS AID UNDERSTANDING OF HELIUM ATOM

Despite the advances in atomic physics in the past 100 years, no scientist has been able to solve analytically a problem involving an atom composed of more than two bodies.

In recent months, however, two Old Dominion University physicists, working together with a colleague in Ireland, have made a significant contribution to understanding the helium atom. They have developed a theoretical model to explain physical results obtained in the 1990s by German physicists, but which heretofore had resisted explanation.

The GWW theory team-the initials coming from the scientists' last names-is made up of Alex Godunov, ODU research assistant professor, Colm T. Whelan, ODU eminent scholar and chair of the physics department, and Professor H.R.J. Walters of The Queen's University, Belfast.

Their work is likely to impact not only physics, but also chemistry and research in superconductivity.

The two bodies of the hydrogen atom-a proton (or nucleus) and one electron-have submitted to a solution, but the complications presented by a helium atom with one alpha particle (nucleus) and two electrons have not been overcome. For a crude analogy relating to the electrons, think of a mother facing the task of keeping track of two hyperactive children. Keeping an eye on one, while challenging, is possible. But add a second kinetic child to the brood and her task becomes overwhelming.

"This is a very major result," said Professor J.H. McGuire of Tulane University, who is the chair of the Division of Atomic and Molecular Physics of the American Physical Society. "The GWW theory from Old Dominion goes to the heart of the problem. The (German) experiments, which are state of the art, have resisted theoretical explanation for some time. This now opens useful, new possibilities for probing correlation in matter, a key, but difficult problem present in various areas of science."

Godunov and the leading German physicist involved in the experiments, Professor Horst Schmidt-Böcking of the University of Frankfurt, have been invited to present the findings at the April meeting in Tampa of the American Physical Society. Joint theory-experiment papers are in press.

Schmidt-Böcking and his team first reported results in 1997 of experiments in which they fired protons at neutral helium, capturing one electron and ionizing the second. It was speculated that the observed experimental results were sensitive to subtle high-order interactions between the two electrons in the target before the collision. However, for nearly eight years, scientists worldwide could not find a theoretical model that was in quantitative agreement with the experiments.

Then the ODU scientists, in collaboration with their Irish colleague, developed their model. The GWW theory arises from an approach that Whelan describes as "orthogonal" to the approaches tried by other scientists. "Everyone else was trying to produce a model of the original Frankfurt data that could explain it in terms of correlation. Our idea was to assume correlation was the dominant effect, and then to develop the simplest model consistent with our understanding of the physics, and use it to predict where one might observe experimental effects, which would show an unambiguous dependence on these subtle correlation effects."

Schmidt-Böcking set out almost immediately to test the GWW theory, and the two teams worked in tandem. "We supplied theoretical guidance," Whelan said. "We took into account what the limitations of their experiments were and redid our calculations to fit within their constraints."

In the end, Whelan said, "They found what we predicted, exactly where we predicted it."

The GWW paper is titled "Fully differential cross sections for transfer ionization-a sensitive probe of high level correlation effects in atoms."

Correlation in matter is important in any system with more than two particles and where there is collective rather than independent particle behavior. Whelan said the task ahead for physicists is to "see how far we can drive this, to look at different ways of describing a helium atom before moving on to other systems."

Godunov, who received his doctoral degree from Moscow State University in 1984, has research interests in atomic collisions, dynamic electron correlation, atomic spectroscopy and computational physics. Before coming to the United States in 1999, he was a leading scientific researcher at the Kurchatov Institute in Moscow. During his time in Russia he won several major awards for his research, including the Kurchatov Prize in 1991 and a Presidential Medal which he received from President Yeltsin in 1997.

Whelan joined the ODU faculty in 2001. Before that, he had led the atomic physics group at the University of Cambridge, where he was a fellow of Magdalene College. He is the editor of five books and more than 140 research papers. He has held visiting positions at Harvard and major universities in the cities of Brest, Frankfurt am Main, Metz, Sendai and Waterloo. He is a fellow of the Institute of Physics and was elected in 2000 a fellow of the American Physical Society.

Walters, professor of theoretical physics at the Queen's University Belfast, is a fellow of the Institute of Physics and a member of the Royal Irish Academy.