SLAC's high-speed "electronic camera" films molecular film in HD

Thanks to an extremely fast "electronic camera" at SLAC, the National Accelerator Laboratory of the Department of Energy, researchers have realized the first high-definition "film" of ring shaped molecules. breaking under the effect of light. The results could help us better understand similar reactions that play a vital role in chemistry, such as the production of vitamin D in our body.

A previous molecular film of the same reaction, produced with SLAC's Linac Coherent Light Source (SLAC) X-ray laser, recorded for the first time the major structural changes that occurred during the reaction. Now, using the laboratory's high-speed electronic diffraction instrument (EDU), these new results provide high-resolution details, showing for example how a bond in the ring breaks and atoms tremble for long periods.

"The details of this initial reaction have now been settled," said Thomas Wolf, scientist at Stanford Pulse Institute of SLAC and Stanford University and head of the research team. "The fact that we can now directly measure changes in binding distances during chemical reactions allows us to ask new questions about the fundamental processes stimulated by light."

Mike Minitti, a scientist with SLAC and involved in both studies, said, "The results demonstrate the complementarity of our unique instruments for the study of ultra-fast processes." One-billionth of a second, UED improves the spatial resolution of these This is an excellent result and the studies validate the conclusions of the other, which is important for the use of entirely new measurement tools. "

Mike Dunne, Director of LCLS, said: "We are now making SLAC's UED instrument accessible to the entire scientific community, in addition to building LCLS's extraordinary capabilities by doubling its power output and increasing its capacity. transforming its repetition rate.The combination of these two tools allow for the best possible studies of fundamental processes at ultra-small and ultra-fast scales. "

The team reported its results today at Nature Chemistry.

HD molecular film

This particular reaction has already been studied several times: when a ring-shaped molecule called 1,3-cyclohexadiene (CHD) absorbs light, a bond breaks and the molecule unfolds to form the almost linear molecule called 1,3,5-hexatriene. (HT). This process is a classic example of ring opening reactions and serves as a simplified model for the study of light-induced processes during vitamin D synthesis.

In 2015, researchers studied the reaction with LCLS, which resulted in the first detailed molecular film of its kind and revealed how the molecule went from a ring to a shape similar to that of a cigar after have been hit by a laser flash. Snapshots, which initially had limited spatial resolution, were developed by computer simulations.

The new study used the UED technique – a technique in which researchers send a high-energy electron beam, measured in millions of electronvolts (MeV), via a sample – to accurately measure distances between pairs of atoms. Taking snapshots of these distances at different intervals after an initial laser flash and tracking their evolution allows scientists to create a stop-motion film of light-induced structural changes in the sample.

The electron beam also produces powerful signals for highly diluted samples, such as CHD gas used in the study, said SLAC scientist, Xijie Wang, director of the MeV-instrument UED. "This allowed us to track the opening reaction of the circle over much longer periods than before."

Surprising details

The new data revealed several surprising details about the reaction.

They showed that atomic motions accelerated as the CHD ring broke, helping molecules to get rid of excess energy and accelerating their transition to the elongated HT form.

The film also showed how the two ends of the HT molecule mutilated while the molecules became more and more linear. These rotational movements lasted at least one picosecond, or one trillion of a second.

"I never would have thought these motions would last that long," Wolf said. "This demonstrates that the reaction does not end with the opening of the ring and that there is much more movement in the light-induced processes than previously thought." . "

A method with potential

Scientists have also used their experimental data to validate a recently developed computer approach to include atomic nuclei motions in chemical process simulations.

"UED provided us with data showing the high spatial resolution needed to test these methods," said Todd Martinez, a Stanford chemistry professor and researcher at PULSE, whose group led the computer analysis. "This article constitutes the most direct test of our methods and our results are in excellent agreement with the experience."

In addition to advancing the predictive power of computer simulations, the results will help us to deepen our understanding of the fundamental chemical reactions of life. Wolf said: "We hope our method will pave the way for more complex molecule studies than those used in life processes."