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3-D Spectroscopy Probes Molecules

Photonics Spectra
Jan 2001
Hank Hogan

Researchers at the University of Illinois at Urbana-Champaign are seeing what's shaking -- literally. Chemistry professor Dana Dlott and fellow researchers John Deàk and Lawrence Iwaki are investigating how vibrational energy flows through molecules on femtosecond time scales. Such three-dimensional vibrational spectroscopy could help probe the workings of micromachines, both natural and man-made.

The group has developed a tunable optical parametric amplifier with unique features that enhance Raman spectroscopy. The device combines the output of a picosecond Ti:sapphire laser with that of an Nd:YAG laser to produce visible pulses at 532 nm and powerful subpicosecond infrared pulses at 2.7 to 3.6 µm.

Alone, neither infrared absorption nor Raman scattering reveals how the various parts of a molecule respond to being wiggled. By combining the two, however, Dlott and his colleagues can watch in real time how vibrational energy flows from one part of a molecule to another.

The researchers first illuminate the molecule under investigation with short pulses of infrared energy that excite specific modes of molecular vibration. Detecting incoherent anti-Stokes Raman scattering with a CCD array, they then observe how that vibrational energy flows throughout the molecule and into other molecules, information that reveals the connections and interactions between the moving parts of the molecule. By tuning the optical parametric amplifier to different wavelengths, the researchers can obtain what amounts to a video library detailing the inner workings of the molecular machine.

Exposing Micromachines

Thus far, the group has studied water and methyl alcohol with the method. Dlott hopes that the research eventually will produce an analytical technique that can uncover the inner workings of biological and man-made micromachines.

This could answer fundamental questions, Dlott explained. "The crankshaft turns the wheels on an automobile," he said, "but if you have some molecular shaft and it turns, what's it going to turn? How's it going to work?"

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