Chemist's Tabletop Technology Sneaks a Peek at Atomic Motion

By Wendy Y. Lawton

To see the work of chemist Christoph Rose-Petruck, you unlock a lab door, don protective goggles, and part a thick, black rubber curtain to reveal an assemblage of lasers, mirrors, X-rays, and infrared lights laid out on a cafeteria-sized table.

The contraption looks like a cosmic pinball machine but functions more like a camera. Called a tabletop ultra-fast laser-driven plasma X-ray spectrometer, it uses extremely fast laser and X-ray pulses to capture the motion of atoms during chemical reactions.

The technology is on the leading edge of a burgeoning field called femtochemistry, the study of chemical reactions as they occur. This ability to take "snapshots" of atoms in motion is revolutionizing chemistry and could have all sorts of consumer applications. Along with colleague Gerald Diebold, Rose-Petruck is developing high-definition mammography with the technology.

"Right now, we're at the beginning. We're at the fundamental level of scientific understanding," said Rose-Petruck, an associate professor in the Department of Chemistry. "We don't know where the technology -- and the knowledge -- will take us. But for a chemist, the work is fascinating. Ultra-fast motions are the essence of every chemical process."

The "before" and "after" of chemical reactions is well understood. Put hydrogen and oxygen together and get water; put iron and oxygen together and get rust.

But what about the in-between? What happens when two or more substances are thrown together? How do the atoms bond, break apart or simply reconfigure to make something new?

This transformation can happen almost incomprehensibly fast. The time it takes for an atom in a molecule to perform one vibration is typically 10 to 100 femtoseconds. (A femtosecond is one quadrillionth of a second.)

Rose-Petruck's equipment operates on the slightly larger picosecond scale, capturing movements that occur in one trillionth of a second.

"Let's put it this way," Rose-Petruck explains. "Light can travel from here to the moon in one second. So picture light leaving this computer at the start of one of our experiments. It wouldn't even make it through the screen before the experiment is over."

Scientists have never seen, in real time, atomic motions that occur during chemical reactions because wavelengths of light are too big to capture images at the atomic scale. But powerful X-rays and ultra-fast lasers help.

Here is how this equipment is put to use in Rose-Petruck's lab.

He places a vial of liquid, which contains the solution he's studying, on the tabletop. A laser light pulse starts the chemical process. A moment later, another laser light pulse shoots through a tube, is reflected by a mirror, then is focused on a steel wire that emits radiation like a strobe, making ultra-fast flashes. The X-rays penetrate the solution and are detected by a camera. The result is a series of measurements of the distance between atoms as they rearrange.

Think of Rose-Petruck's "camera" taking single-frame snapshots that can be strung together to show movement over time. "In some sense, we're in the movie business," he says. "But our movies are just very short."

The chemists don't obtain actual images. Instead, they obtain data used to graph motion. Rose-Petruck uses that data to make animations. One shows what appears to be strings of pearls -- representing rows of atoms -- hanging close together. Each one crashes against a neighbor in a wave-like, slow-motion chain reaction.

Last year, Rose-Petruck and members of his research lab took the first measurements of transition metal complexes using a tabletop ultra-fast laser-driven plasma X-ray source. The results were published in March in the Journal of Chemical Physics and created a scientific buzz. Rose-Petruck has presented his findings to chemists and physicists at Harvard, Stanford, MIT, and the University of California-Berkeley.