Most of what is known about chemical bonds comes from studying molecules at rest. But when molecules absorb light, their constituent atoms start vibrating, which rapidly changes the shape of their chemical bonds. This can dramatically alter how molecules react with one another.
Studying the dynamics of atoms on these ultrafast timescales has been difficult, but in recent years, new X-ray sources have opened up new possibilities. At the Linac Coherent Light Source (LCLS) X-ray free-electron laser at SLAC National Accelerator Laboratory in Menlo Park, California, scientists have developed an innovative technique called high-energy-resolution off-resonant spectroscopy, or HEROS.
It involves passing high-energy electrons that have been synchronized with a laser pulse through the oscillating molecules and then analyzing how the electrons are scattered away at various angles. This allows researchers to directly observe how the lengths and angles of the molecule's chemical bonds are changing in real-time.
In a proof-of-concept experiment, the team studied carbon monoxide molecules hit by a femtosecond laser pulse at SLAC. The experiments measured, in real time, the time-dependent changes in the carbon-oxygen bond length after light was absorbed.
"We want to understand how energy flows between different parts of the molecule," said Giulia Pinardi, a postdoctoral researcher at SLAC and the lead author of a study published in Physical Review Letters on December 17. "If that can happen on a very short timescale, it can influence what the molecule ends up doing."
In this case, carbon monoxide vibrates after light absorption, which prevents the molecule from dissociating into free carbon and oxygen atoms. By capturing this motion in detail, the team could learn a great deal about how molecular vibrations affect chemical reactivity.
In the future, the team plans to use the HEROS technique to probe more specific molecular motions. They also want to follow chemical reactions in more complex molecules that could be relevant for the design of new drugs or materials.
"HEROS is essentially like strobe photography," said co-author Mike Minitti. "We can take a series of snapshots with an X-ray laser to watch the motions as a reaction goes forward. That's something new and is a testament to the X-ray laser."
