Laser Technology Used to Reveal Secrets of Molecular Motion in Supercooled Liquids

Contact: Mary Lenz(512) 471-3151Date: April 12, 2001

Supercool molecules: UT Austin researchers use laser technology to reveal secrets of molecular motion in supercooled liquids

AUSTIN, Texas -- Researchers at The University of Texas at Austin have used a new form of laser technology called single molecule spectroscopy to make important contributions to understanding the motion of molecules in supercooled liquids, a problem of interest to scientists for more than half a century. Their research will be published in the April 13 issue of the journal Science.

The researchers are Laura A. Deschenes, a graduate student in the UT Austin department of chemistry and biochemistry, and Dr. David A. Vanden Bout, an assistant professor in the department and a member of the Center for Nano and Molecular Science and Technology in the UT Austin Texas Materials Institute.

Vanden Bout specializes in research using light to probe the microscopic structure of materials. He was designated a Research Corporation Cottrell Scholar last year, and received the Camille and Henry Dreyfus New Faculty Award in 1997. Deschenes, a graduate of Boston College with a degree in chemistry, is the lead author of the paper and a UT Austin Ph.D. candidate.

The Science article announces both the discovery of a successful method for observing the movement of individual molecules as well as new insights into molecular motion in a material just before it turns into a glass.

When liquids freeze to become solid, the molecules arrange themselves into organized structures. Supercooled liquids, in contrast, are cooled so fast they go below freezing before the molecules have a chance to organize.

"If you lower the temperature of a supercooled liquid, the molecules move slower and slower until eventually the material is so cold all motion has stopped," Vanden Bout explained. "Then you have a different kind of solid -- you have a glass, an amorphous solid. If you look at the motions of molecules in supercooled liquid, you discover they do not look like the motions of molecules in regular liquid."

The researchers focused on the rotation of molecules just before this glass transition. In a normal liquid, all the molecules rotate at the same rate, which means only one time scale is involved. The researchers wanted to find out what happens to the rotation of molecules in a supercooled liquid and whether all molecules in a supercooled form were rotating in the same way at the same time, or whether each molecule was rotating at a different rate.

The researchers made refinements in previously used laser technology to isolate and illuminate a very bright orange molecule called Rhodamine 6G. Because the method allowed molecules to be observed individually, the researchers were able to avoid the jumbling effect that masks what is really happening. "If you look at all the molecules at the same time, you can't tell the difference in their movements. You get a jumble of times," Vanden Bout said.

"What we discovered is that the rotations of individual molecules are all very different from each other. Some of the molecules are moving really fast, and some are really slow. In our supercooled liquid, there are regions within the liquid that are different from one another," Vanden Bout said. "Essentially, the supercooled liquid is composed of small, liquid-like domains moving at different rates. Within each domain, the motion looked like the motion of normal liquid."

Vanden Bout said researchers also discovered that "the domains are not static. We can watch them interchange with one another. When a fast-moving molecule switches environments, it becomes slow."

The Texas Materials Institute is an interdisciplinary research unit of UT Austin involving the fields of modern materials science and engineering. Research areas include design, synthesis, characterization and fabrication of new or improved materials that can be used in structural, microelectronic, magnetic, dielectric and optical devices. Research areas also include nanostructure materials for mechanical, superconductor and optical applications, structural mechanics and alternate methods of energy conversion and storage.

For more information, contact Dr. David Vanden Bout at (512) 232-2824 or Caroline Ladhani at the College of Natural Sciences at (512) 232-1075, or http://www.cm.utexas.edu/faculty/VandenBout.html. The Texas Materials Institute Web site at http://www.utexas.edu/academic/tmi/. The Center for Nano and Molecular Science and Technology Web site is http://www.cm.utexas.edu/cnm.

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