Using the powerful LCLS X-ray laser, they directly imaged for the first time how molecules rearranged during a chemical reaction controlled by light.
Key takeaways:
- For the first time, researchers have imaged a molecule undergoing a coherently controlled chemical reaction – a reaction steered with pulses of laser light.
- Powerful X-ray pulses from the LCLS are routinely used to visualize how a molecule’s atoms rearrange in real time during a reaction initiated by a laser pulse, but here, with the help of a novel analysis method, the researchers imaged a reaction that was controlled by a third pulse.
- This approach could help researchers understand and eventually control light-activated reactions in photochemistry, catalysis, and light-responsive materials.
Since the 1980s, researchers have sought to use laser light to control chemical reactions relevant to photochemistry, catalysis and light-responsive materials. But this technique, known as coherent control, has a blind spot: There hasn’t been a way to directly see the molecules in these reactions as their structures rearrange.
Now, researchers at the Department of Energy’s SLAC National Accelerator Laboratory have imaged a coherently controlled chemical reaction for the first time. Their work, published in Physical Review A, uses ultrafast X-rays from the Linac Coherent Light Source (LCLS) to show in real time how atoms move in a molecule that was excited and manipulated with laser light.
“There are many challenges with controlling chemical reactions, but seeing is believing,” said study lead author Tom Hopper, assistant professor at the University of Central Florida who was a postdoc at SLAC at the time of the study. “If you can see something directly, it opens up a new level of control.”
Illuminating a blind spot
What makes coherent control so tricky is that the molecule being manipulated with laser light will eventually deviate from the desired pathway. If researchers can see the molecule’s structural evolution in real time, they can start to put together a picture of when and how this happens, which may help them figure out how to prevent it.
One of the simplest and most successful methods of coherent control is the “pump-dump scheme.” This involves hitting a molecule with two laser pulses: A “pump” pulse first excites the molecule, initiating the reaction, followed by a “dump” pulse that nudges the reaction down a certain pathway.
Read more on the SLAC website
Image: This illustration shows a pump–dump–probe sequence of ultrafast laser and X-ray pulses used to control and image a chemical reaction. The pump laser pulse excites molecules within a sample and initiates a chemical reaction. It is followed by a second laser pulse, the dump pulse, that nudges the reaction down a certain pathway. Finally, an X-ray probe pulse traverses the sample at varying stages of the chemical reaction. The scattered X-rays create diffraction patterns on a detector (at right). The changing patterns contain information about the molecular structure and how it evolves during the reaction.
Credit: Greg Stewart/SLAC National Accelerator Laboratory