March 12, 2012
J.R. MacDonald laboratory physicists help capture first image of atomic motions in a chemical reaction
Physicists at Kansas State University have collaborated to formulate a technique that records the first image of two atoms bonding together to form a molecule.
In addition to capturing a molecule's formation, the feat is the first step to enabling scientists, not nature, to control chemical reactions at a molecular level.
Chii-Dong Lin, distinguished professor of physics, and Junliang Xu, doctoral graduate student in physics, Hefei, China, are both researchers in the university's James R. Macdonald Laboratory. They collaborated with physicists from Ohio State University to visually document a molecule's formation. Their study, "Imaging ultrafast molecular dynamics with laser-induced electron diffraction," was published in a recent issue of Nature and was funded by the U.S. Department of Energy.
In order to capture the atomic-sized event the researchers used a technique called laser-induced electron diffraction, or LIED, which is typically used to study gas-phase molecules. The researchers fired an ultrafast laser pulse at molecules whose atoms were bonding together. The laser's electric field ripped an electron out of the molecule before the oscillating electric field changed the electron's direction. This fired the electron back into its mother molecule and illuminated the molecule. The electron-created camera flash allowed researchers to record the event.
After examining the image, physicists were surprised to find that the bond length between the two oxygen atoms shrank by 10 percent, or 0.01 nanometers. That meant that when an electron was removed from the molecule, the two atoms readjusted their distance in a matter of femtoseconds -- one-quadrillionth of a second. The LIED technique captured such a small change, making it the first technique to determine the position and time of a chemical event at such high precision and timescale, Lin said.
"With the traditional method of using X-ray diffraction and electron diffraction we could not study this subtle change," Lin said. "The beams were not short enough. It would be like using a sundial to measure a person running 100 meters. Yes, you are using a clock, but it's too slow to be accurate."
For the study, researchers used simple oxygen molecules to test the LIED technique. The molecules were chosen because scientists believe they know everything about them, Lin said.
With the LIED technique, images are taken by firing an electron -- a tiny quantum bullet in this case -- against a moving target. The electron's wave nature produced the image, not unlike a flashlight or X-ray, Lin said.
"In the old days when a bullet leaves the gun, gravity decides its path and it cannot be redirected," Lin said. "With LIED, scientists can control how and when the tiny electron bullets are fired by manipulating the lasers. Once the techniques are fine-tuned, we control how the molecules react and what products are formed, not nature.
"In the physical sciences, like physics and chemistry, we want to be able to control energy and matter," Lin said. "That is our goal. We don't want to just let nature decide where a particle or molecule goes. As scientists we want to make that decision."
Kansas State University formulated the LIED idea. The experimental proof was carried out at Ohio State University, which had the necessary tools to perform the experiment.
"Now that LIED has been proven, the race is on to make it work for more complex molecules," Lin said.