A Blink Of Technology: Femtosecond Chemistry Redefining Molecular Interactions

PITTSBURGH—Using ultrafast optics and lasers, physicists and chemists are opening a portal through which they can view the subtlest and quickest changes in atomic motions.

Understanding these ultrasmall, ultrafast changes could lead to new avenues for controlling chemical reactions at surfaces, say scientists at the National Institute of Standards and Technology.

Speaking at the American Physical Society's annual meeting, NIST surface scientists J. William Gadzuk and Richard R. Cavanagh explained that the emerging field of femtosecond (quadrillionth of a second) chemistry could enable scientists to break chemical bonds selectively, spur reactions and choose desired products.

"We're looking for ways to get around traditional thermal chemistry. We're trying to control atomic motion to get what we want," Cavanagh said. "It would be a more efficient way of driving a reaction."

Scientists are using precisely timed laser pulses to see femtosecond changes in various parts of atoms and molecules. Measuring the changes in femtoseconds gives scientists a glimpse of the various stages atoms go through in a reaction.

Just as a photographer uses a shutter faster than his subjects to freeze action in an Olympic race, scientists have devised a very fast shutter based on the speed of light to measure molecular motions. Laser pulses, which travel approximately a billion feet per second, can freeze action in atoms that vibrate about as fast as sound travels.

In femtosecond surface chemistry, light is also used to initiate a reaction by exciting electrons. Hot electrons so produced can redistribute the energy of the laser pulse into preselected parts of the molecular surface system.

"We're talking about being able to strip out pieces and put them wherever you would like," explained Gadzuk. "It's a powerful new frontier." Gadzuk, a theoretician in NIST's Surface and Microanalysis Science Division, presented APS members with an overview and history of femtosecond chemistry at surfaces with particular emphasis on wave packet modeling carried out at NIST.

"Much of what is today called selective femtochemistry owes its existence to ultrafast laser technology capable of producing pulses comparable to the time scale of molecular vibrations," Gadzuk said.

Two such laser pulses can create a shutter for viewing atomic motion. The pulses are aimed at a sample simultaneously, but one is delayed by mirrors and arrives at the sample shortly after the first. Interactions between the pulses and the sample show how atoms in the sample changed between the first and second pulse.

Cavanagh and postdoctoral researcher Thomas Germer are using fast optics and lasers to test new femtochemistry theories. In his APS presentation, Cavanagh described recent experiments to test the coupling strength of bonds between carbon monoxide molecules and copper surfaces.

The aim of this research was to discover whether energy localized in electrons or in atoms has a greater influence on chemical reactions. The NIST experiments charted very fast temperature changes in electrons after they were struck with photons. They also measured how quickly the nuclei of atoms changed temperature when struck with fast laser pulses.

The experiments are helping scientists understand how quickly energy is transferred between different molecules on a surface. Studies of the carbon monoxide and copper bonds reveal that the temperature of both electrons and atoms influences chemical reactions. "Our experiments show that both are important, occurring in a few picoseconds (trillionths of a second)," Cavanagh said.

Experiments such as these are helping scientists to determine the optimal amount of light, or energy, to trigger a chemical change. As femtosecond chemistry reveals more about atomic particle behavior, scientists may devise futuristic ways of synthesizing materials or fabricating nanostructures.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.