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Killer pulses help characterize special surfaces
Date:7/29/2008

CHAMPAIGN, Ill. Detecting deadly fumes in subways, toxic gases in chemical spills, and hidden explosives in baggage is becoming easier and more efficient with a measurement technique called surface-enhanced Raman scattering. To further improve the technique's sensitivity, scientists must design better scattering surfaces, and more effective ways of evaluating them.

Researchers at the University of Illinois, led by chemistry professor Dana Dlott, have devised a method to evaluate substrate surfaces by using a series of killer laser pulses. They describe the method and report measurements for a commonly used substrate in the July 18 issue of the journal Science.

Surface-enhanced Raman scattering, which functions by adsorbing molecules of interest onto rough metal surfaces, typically enhances the Raman spectrum a million times. Hot spots can occur, however, where the electric field enhancement can be a billion or more.

Current surface characterization techniques cannot tell hot spots from cold spots, and create an average value across the entire substrate surface.

"Looking at a spectrum, you can't tell if it's the result of a small number of molecules in hot spots or a large number of molecules in cold spots," Dlott said. "Two materials could have the same average spectrum, but behave quite differently."

Dlott, graduate student Ying Fang and postdoctoral research associate Nak-Hyun Seong came up with a way to measure the distribution of site enhancements on the substrate surface. Using killer laser pulses, their technique can count how many molecules are sitting in the hottest spots, how many are sitting in the coldest spots, and how many are sitting between the two extremes.

The killer pulse is a short duration laser pulse with a variable electric field. When the electric field is strong enough, it rips a molecule apart, "killing" it.

"If a molecule is in a very hot spot on the substrate, where the elect
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Contact: James E. Kloeppel
kloeppel@illinois.edu
217-244-1073
University of Illinois at Urbana-Champaign
Source:Eurekalert

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