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Researchers Probe for Good Vibrations

Photonics Spectra
May 2000
Hank Hogan

Researchers at the University of Wisconsin-Madison have found that if one laser beam is a good research tool, two are even better. Working together, two infrared lasers can probe relationships between molecular vibrational states that previously were difficult to detect. The result may be a clearer understanding of intra- and intermolecular interactions.

John C. Wright, a chemistry professor, and Wei Zhao, a postdoctoral assistant, developed the technique, which is described in detail in the Feb. 14 issue of Physical Review Letters. Wright noted that the approach could overcome some longstanding difficulties in molecular spectroscopy. "It has the advantage that it can eliminate the spectral congestion and selectively enhance specific components in complex materials. These problems have been the Achilles' heel for vibrational spectroscopy in the past," he said.

Researchers at the University of Wisconsin-Madison have developed a laser technique that may provide a better understanding of intra- and intermolecular interactions.

In this technique, which the researchers call doubly vibrationally enhanced four-wave mixing, two infrared laser beams of differing frequencies are focused onto a sample. The wavelengths of light selected are a function of the material and the vibrational mode being investigated. The beams are at an angle to each other to optimize the resulting output.

A molecule vibrates because of the springy nature of the bonds that hold it together. The appropriate wavelength of light pumps up particular molecular vibrations, which in turn create a detectable spectroscopic signal. The problem is that a molecule may have many such springlike bonds. So the output signal can be a conglomeration of several possible bonds.

In this new approach, as the two beams bombard the molecules, they enhance the vibrational states that impact one another. This information is of prime interest to biology and chemistry researchers, but until now good tools to extract such data were nonexistent.

Wright noted that over the short term the need will be for fundamental work to understand doubly vibrationally enhanced spectroscopy. Over the long term, there are many possible applications. "One would imagine that [doubly vibrationally enhanced] methods should be applicable for any work that is currently using infrared absorption or Raman spectroscopy," he said.

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