For decades, chemists have dreamed of steering chemical reactions with light—not just watching them happen, but actively guiding molecules down specific pathways like a conductor shaping a symphony. Now, researchers at the Department of Energy's SLAC National Accelerator Laboratory have done something remarkable: they watched it happen in real time.
Publishing their findings in Physical Review A, the team used ultrafast X-rays from the Linac Coherent Light Source (LCLS) to image a coherently controlled chemical reaction for the very first time. The breakthrough fills a gap that has frustrated researchers since the 1980s.
"There are many challenges with controlling chemical reactions, but seeing is believing," said study lead author Tom Hopper, an assistant professor at the University of Central Florida who was a postdoctoral researcher at SLAC when the work was conducted. "If you can see something directly, it opens up a new level of control."
The technique, known as coherent control, relies on hitting molecules with carefully timed laser pulses. A "pump" pulse excites the molecule and kicks off a reaction; a "dump" pulse then nudges that reaction down a particular pathway—either calming the molecule back down or pushing it further until chemical bonds break. The challenge has always been that molecules eventually stray from their intended paths, and researchers had no way to see exactly when and how that deviation happened.
Traditionally, scientists inferred structural information using spectroscopy, which Hopper compared to "a flashlight that only illuminates part of the reaction." The SLAC team took a different approach: they added a third pulse—ultrafast X-rays—that probed the molecule's structure directly, revealing atomic movements as they unfolded.
To demonstrate their method, the researchers worked with iodine vapor, where two iodine atoms are bonded together. By adjusting the timing of the dump pulse, they could either return the molecule to a lower energy state or cause the bond to snap, sending the atoms flying apart. Diatomic iodine was ideal for this proof-of-concept because its relatively simple structure and heavy atoms scatter X-rays strongly, making the signals easier to detect.
Adi Natan, a staff scientist at SLAC's Stanford PULSE Institute who developed the new analysis method for transforming X-ray data into structural information, described the significance: "With X-ray scattering, you can see everything. It allows you to have a better understanding of what happens and what the limits of our methods are. We can uncover that blind spot."
The implications extend far beyond iodine. The team anticipates applying this approach to more complex coherently controlled reactions, and they expect it to become even more powerful once the high-energy upgrade to LCLS is complete. Hopper's group also plans to explore coherent control in materials rather than isolated molecules.
"I'm sure that work will be done in conjunction with SLAC," Hopper said. "They have the best X-ray source in the world and the expertise to interpret the data as it's coming out of the experiment."