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Images capture molecular motion in real time

Caught in the act: Images capture the molecular movements in real time

The researchers photographed the subtle movements of a molecule called N-methyl morpholine when it is excited by UV light. Credit: Brown University / SLAC

The researchers used ultra-high-speed X-ray pulses to make a high-resolution "film" of a molecule subjected to structural motions. The research, published in Nature Chemistry, reveals the dynamics of processes with unprecedented details – capturing the excitation of a single electron in the molecule.

The ability to see molecular motions in real time offers insight into the processes of chemical dynamics unthinkable just a few decades ago, and could potentially help optimize reactions and design new types of chemistry.

"For many years, chemists have learned about the existence of chemical reactions by essentially studying the molecules present before and after a reaction," said Brian Stankus, a recent PhD in philosophy. graduated from Brown University and co-authored the paper. "It was impossible to observe chemistry as it occurs because most of the molecular transformations occur very quickly, but ultra-fast light sources like the one used in this experiment allowed us to measure Molecular movements in real time.This is the first time subtle effects have been seen with such clarity in an organic molecule of this size ".

The work is a collaboration between Brown chemists, scientists from the SLAC National Accelerator Laboratory and theoretical chemists from the University of Edinburgh in the UK. The team was led by Peter Weber, professor of chemistry at Brown.

For this study, the researchers examined the molecular motions that occur when the organic N-methyl morpholine molecule is excited by pulses of ultraviolet light. The X-ray pulses of the SLAC coherent light source (LCLS) were used to take snapshots at different stages of the dynamic response of the molecule.

"We basically hit the molecules with UV light, which triggers the response, then a few fractions of a second later, we take an" image "- we actually capture a scattering pattern – with an X-ray pulse", a Stankus said. "We repeat this again and again, with different intervals between the UV pulse and the X-ray pulse to create a time series."

X-rays diffuse particular patterns depending on the structure of the molecules. These motifs are analyzed and used to reconstruct a shape of the molecule as the molecular movements unfold. This trend analysis was conducted by Haiwang Yong, a graduate student of Brown and co-lead author of the study.

The experiment revealed an extremely subtle reaction in which a single electron is excited, causing a distinct pattern of molecular vibrations. The researchers were able to visualize in detail the electronic excitation and the atomic vibration.

"This document is a real milestone because we were able to measure for the first time with great clarity the structure of a molecule in an excited state and with a temporal resolution," said Weber, the corresponding author. of the study.

"Performing these types of measurements almost noiselessly, both in terms of energy and time, is not a trivial matter," said Mike Minitti, senior researcher at SLAC and co-author of the report. ;study. "Over the past seven years, our collaboration has learned a great deal about the best use of the various LCLS diagnostics to accurately measure small X-ray intensity fluctuations and, to a greater extent, to track changes in X-ray intensity. Femtosecond Time Scale and Evolution of Molecules All of this has been the basis for the development of custom data analysis routines that virtually eliminate the annoying and unwanted signals from our data, and these results demonstrate the fidelity we can get. "

The researchers explain that a particularly interesting aspect of the reaction is its consistency. In other words, when groups of these molecules interact with light, their atoms vibrate together.

"If we can use experiments like this to study how exactly light can be used to drive the collective movement of billions of molecules, we can design systems that can be controlled in a consistent way," said Stankus. "In simple terms: if we understand exactly how light directs molecular motions, we can design new systems and control them to achieve useful chemistry."

Watch molecules separate in real time

More information:
Brian Stankus et al., Ultrafast X-ray scattering reveals a vibrational consistency following Rydberg's excitation. Nature Chemistry (2019). DOI: 10.1038 / s41557-019-0291-0

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