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| Rotational Dynamics Theory | |
In the mid-1980s, former Georgia Tech physics professor Dr.
William G. Harter and colleague Dr. Chris Patterson of Los Alamos
National Laboratory advanced their theory of the rotational
dynamics of molecules. The "semi-classical" theory incorporates
certain classical features into the quantum description of the
behavior of matter.
Harter and Patterson asserted that molecular motion uses a
rotational energy surface, which is a way of generalizing to
rotations the potential energy surface commonly used to describe
vibrations. The rotational energy surface allows scientists to
define the rotational-vibrational quantum state of a molecule
more clearly and to identify the values or range of values for
each of the relevant physical quantities.
By 1985, Harter and Patterson had examined molecular data
obtained with a tunable laser and discovered that molecular
rotation resembles just what its name implies — the rotation of
a planet on its axis. As molecules spin around their center of
gravity, they wobble in a conical pattern or "precess" as they
rotate around a multitude of axes. Also, molecules execute a
generally slower "tunneling" or tumbling motion that would be
forbidden in a world governed by classical mechanics. This
rotational form of so-called "quantum tunneling" is seen in
nuclear decay and electron flow in semiconductors.
Crucial to the model is that as the molecule rotates,
precesses and tunnels, centrifugal force is exerted on its
nuclei. This "stretches out" the electronic bonds of the
molecule, affecting the vibrational motion of the molecule —
which in turn affects its rotation and precession, as well as the
dynamics of electronic and nuclear spin moments. The net result
is an extremely complex series of interacting movements, which
the researchers portrayed in a relatively simple way using the
rotational energy surface described in their theory.
In 1987, Harter and former Georgia Tech graduate student David
Weeks used Harter's theory to do the first predictions on the
rotational-vibrational spectra of the soccer ball-shaped molecule
Buckminsterfullerene (C60), nicknamed "buckyball." This structure
had been proposed in 1985 by a group of Rice University
researchers, who had seen a mass-spectra peak of atomic mass 720.
Subsequently, researchers from the University of Arizona and the
Max Planck Institute used Harter and Weeks' findings and their
Macintosh software program to further analyze C60. In 1989, those
researchers realized from Harter and Weeks' vibrational spectral
predictions that they had been making C60 since the early 1970s.
Other experts were skeptical, but IBM labs at San Jose, Calif.,
verified the University of Arizona's results in 1990. Just two
years later, Science named C60 "Molecule of the Year," and the
Rice University-led research team received a Nobel prize in
chemistry in 1996 for its work with the molecule.
Harter is now a professor of physics at the University of
Arkansas, where he studies optimal control theory for quantum
systems. In 1995, he was elected a fellow of the American
Physical Society. Weeks is a professor at the U.S. Air Force
Postgraduate School near Wright Patterson AFB in Dayton, Ohio.
Copyright Boris Pevzner, 1995
Former Georgia Tech physics professor Dr. William Harter
proposed a molecular rotational dynamics theory he used to make
the first predictions on the rotational-vibrational spectra of
the soccer ball-shaped molecule Buckminsterfullerene (C60),
nicknamed "buckyball."
For more information, contact Dr. William Harter at
wharter@comp.uark.edu
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