Using a high-speed “electronic camera” in the National SLAC Accelerator Laboratory of the Ministry of Energy, scientists simultaneously capture the motions of electrons and nuclei in a molecule after it is excited by light. This is the first time this has been done with ultrafast electron diffraction, which scatters a powerful beam of electrons from materials to take small molecular motions.
“In this study, we show that with ultra-fast electron diffraction, it is possible to track electron and nuclear changes by naturally separating the two components,” said Todd Martinez, a Stanford professor of chemistry and a researcher at the Stanford PULSE Institute. “This is the first time we’ve been able to see both the detailed positions of atoms and electronic information at the same time.”
The technique can allow researchers to get a more accurate picture of how molecules behave while measuring aspects of electronic behavior that underlie quantum chemistry simulations, providing a new basis for future theoretical and computational methods. The team published its findings today at science,,
Skeletons and glue
In previous studies, the SLAC’s ultra-fast electron diffraction tool, MeV-UED, has allowed researchers to create high-resolution “films” of junction molecules and structural changes that occur when ring-shaped molecules open in response to light. But so far the instrument has not been sensitive to electronic changes in molecules.
“In the past, we’ve been able to track atomic movements as they happened,” said lead author Jie Yang, a scientist at the SLAC Acceleration Directorate and the PULSE Institute in Stanford. “But if you take a closer look, you’ll see that the nuclei and electrons that make up atoms also have a specific role to play. The nuclei make up the skeleton of the molecule, while the electrons are the glue that holds the skeleton together.”
Freezing of ultra-fast movements
In these experiments, a team led by researchers from SLAC and Stanford University studied pyridine, which belongs to a class of ring-shaped molecules that are central to light-controlled processes such as UV-induced DNA damage and repair, photosynthesis and solar energy conversion. Because the molecules absorb light almost instantly, these reactions are extremely fast and difficult to study. Ultra-high-speed cameras such as MeV-UED can “freeze” movements occurring within femtoseconds or millionths of a billionth of a second to allow researchers to track changes as they occur.
First, the researchers passed laser light into a gas of pyridine molecules. They then blew up the excited molecules with a short pulse of high-energy electrons, generating images of rapidly rearranging electrons and atomic nuclei that could be strung together in a stop-motion film of light-induced structural changes in the sample.
The team found that elastic scattering signals produced when electrons scatter a pyridine molecule without absorbing energy encode information about the nuclear behavior of the molecules, while inelastic scattering signals produced when electrons exchange energy with the molecule contain information about electronic changes. . Electrons from these two types of scattering appeared at different angles, which allowed the researchers to clearly separate the two signals and directly observe what the electrons and nuclei of the molecule are doing at the same time.
“Both of these observations agree almost exactly with a simulation that was designed to take into account all possible channels of response,” said co-author Xiaolei Zhu, who was a doctoral student at Stanford during the experiment. “This gives us an extremely clear view of the interaction between electronic and nuclear change.”
The scientists believe that this method will complement the range of structural information collected by X-ray diffraction and other instrument techniques such as Linac’s SLC Coherent Light X-ray Laser (LCLS), which is able to measure accurate details of the chemical dynamics of the most short deadlines, as recently reported for another light-induced chemical reaction.
“We see that MeV-UED is increasingly becoming a tool that complements other techniques,” said co-author and SLAC scientist Thomas Wolf. “The fact that we can obtain electronic and nuclear structures in the same set of data, measured together but observed separately, will provide new opportunities to combine what we have learned with knowledge from other experiments.”
“A new way of looking at things”
In the future, this technique could allow scientists to monitor ultrafast photochemical processes in which the time for electronic and nuclear changes is crucial to the outcome of the reaction.
“This really opens up a new way of looking at things with ultrafast electron diffraction,” said co-author Shidie Wang, director of MeV-UED. “We are always trying to understand how electrons and nuclei actually interact to make these processes so fast. This technique allows us to distinguish which comes first – the change in electrons or the change in nuclei. Once you get a complete picture of how these changes play, you can start predicting and controlling photochemical reactions. ”
First, look directly at how light excites electrons to initiate a chemical reaction
“Simultaneous monitoring of nuclear and electronic dynamics by ultrafast electron diffraction” science (2020). science.sciencemag.org/cgi/doi… 1126 / science.abb2235
Capture the coordinated dance between electrons and nuclei in a light-excited molecule (2020, May 21)
retrieved 22 May 2020
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