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Multidimensional Technique Enhances Vibrational Spectroscopy

CHAMPAIGN, Ill. -- By combining ultrashort pulses from a mid-infrared laser with pulses of visible light, chemists at the University of Illinois have added an important new dimension to vibrational spectroscopy. The new spectroscopic technique allows researchers to investigate vibrational energy redistribution in molecules with unprecedented detail.

"Molecules have specific vibrational motions, which can be used as spectral fingerprints," said Dana Dlott, a UI professor of chemistry. "Our spectroscopic method allows us to monitor vibrational energy flow through a molecule on femtosecond time scales. We can therefore characterize the dynamic mechanical properties of molecules in real time -- which is important in virtually every chemical process and of special interest in the field of nanotechnology, where machines will be the size of molecules."

Ordinary infrared spectroscopy is a one-dimensional technique that is widely used to identify molecules by their spectral fingerprints. By adding both a time dimension and an additional spectral dimension, Dlott and his colleagues -- postdoctoral research associate John Deàk and graduate student Lawrence Iwaki -- have developed a three-dimensional technique that yields much more information. Instead of getting just a fingerprint, they obtain an entire library of "motion pictures."

"When a molecule is laser-pumped by an infrared pulse, it executes a complicated, time-dependent dance," Dlott said. "We can watch that dance by obtaining a two-dimensional time-series of Raman spectra that shows how the vibrational energy flows through the molecule. Then, by tuning the infrared pulse into different parts of the molecule's spectrum, we get different dances. After recording all the possible dances the molecule can execute, we have a complete picture of the molecular mechanics."

Using their technique, the researchers studied two common and important liquids, water and methyl alcohol, with extremely fine detail. "Of particular interest was how the vibrational energy redistribution dance changed as the pump pulse was tuned through the third dimension," Dlott said. "Earlier work by other researchers suggested this dance might not depend much on where the infrared pulse occurs. But we found that it depends a great deal on where we excite the molecules; and more importantly, that measuring the dependence of the vibrational energy redistribution on infrared frequency provides the key to elucidating the fundamental mechanics involved."

Ultimately, by providing femtosecond-resolved snapshots of molecular motions, multidimensional vibrational spectroscopy could allow researchers to learn much more about the mechanics of molecules. This, in turn, could lead to a greatly improved understanding of chemical processes, and even better analytical techniques for studying complicated mixtures of large molecules such as biological systems.

Dlott presented the team's latest experimental results at the American Chemical Society meeting, held Aug. 20-24 in Washington, D.C. A paper describing the technique will appear in the Journal of Physical Chemistry.


Source: University Of Illinois At Urbana-Champaign. September 2000

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