Don't Blink! Ultrafast Diffraction Captures Complex Phenomena

Kathleen G. Tatterson

PASADENA, Calif. -- Scientists at the California Institute of Technology have demonstrated a technique for producing light in femtosecond pulses, giving molecular chemists a peek at previously invisible chemical processes.
Ahmed Zewail and his colleagues at Cal Tech used ultrafast electron diffraction to generate short pulses of bright, ultraviolet, frequency-doubled laser light at 310 nm at approximately 250 µJ to split molecules of di-iodomethane. The team published its results in Nature (March 13, 1997).
Electron diffraction is not new to the study of molecular structure; however, the difficulty of creating such short pulses has made it impossible to study ultrafast structural changes in isolated molecules. Attempts to keep electron bursts less intense would result in scattered pulses that would be too weak to measure.
In the new technique, a colliding-pulse mode-locked ring dye laser sends bursts every 10 fs. The laser sends a long pulse through a series of optics that chop the beam into smaller segments and send it to the reaction chamber via a longer, slower optical path. The Nature paper reports that the team observed the diffraction pattern with the aid of a very sensitive charge-coupled device camera.

Visualizing chemical dynamics
"It's the first time someone has looked at structural changes using these techniques," said Toni Taylor, deputy group leader at Los Alamos National Laboratory in New Mexico. "It could have a significant impact in studying the dynamics of chemical reactions and will enable researchers to look at much more complex molecules than they did before. (The Cal Tech group) has some work to do, but it looks very promising."
According to Zewail, the team is continuing its research with the technique on molecular processes.
"We expect to see some very exciting things in the next few months," Zewail said.