The way molecules interact with light is very important. Sometimes they absorb it and start vibrating, other times it makes them spin, and in certain cases, it breaks the molecules apart. Researchers at the SLAC National Accelerator Laboratory have now produced an incredible atomic movie that focuses right on the moment when molecules may or may not break due to light. The study is published in Science.

Studying the “point of no return” is very important when it comes to understanding chemical reactions. The team shone a laser light on a gas whose molecules were made up of 5 atoms each. The light pulse stretched the bond between the atoms and the molecules ended up at a crossroads: either the bond broke or the atoms started vibrating to keep the bond intact.

“The starting and end points of a chemical reaction are often obvious, but it’s much more challenging to take snapshots of the rapid reaction steps in between,” lead author Dr Jie Yang, from SLAC’s Accelerator Directorate and the Stanford PULSE Institute, said in a statement. “The crossroads where a molecule can do one thing or another are an important factor in determining the outcome of a reaction. Now we’ve been able to observe directly for the first time how the atomic nuclei of a molecule rearrange at such an intersection.”

The molecule studied is called trifluoroiodomethane and is made up of a carbon atom surrounded by three fluorine atoms and an iodine atom. The laser pulse affected the bond between the carbon and the iodine. The team chose this molecule as a “simple” model for more complex light-driven reactions. DNA molecules, for example, can be easily damaged by ultraviolet radiation while other molecules are not. This work provides a few clues as to why.

To take such an incredible snapshot of a reaction researchers used an ultrafast electron diffraction (UED) camera. This device shoots a beam of high-energy electrons and uses them to create still images of the reaction as it evolves. Strung together, they create a movie of the changing molecule.

“UED was absolutely crucial to seeing that point during the reaction,” added Xijie Wang, head of SLAC’s UED program and the study’s principal investigator. “Other methods either don’t detect nuclear motions directly or haven’t reached the resolution necessary to make this kind of observation in gases.”

The next step is studying such changes in a liquid rather than a gas. This would take us closer to understanding how these reactions happen in biological systems.