Ultrafast imaging technology reveals how ozone-depleting molecules react to light

Researchers have observed for the first time how bromoform rearranges its atoms in less than a trillionth of a second after being subjected to an ultraviolet (UV) pulse. The imaging technique captured the long-predicted pathway by which ozone-damaging molecules change their structure upon interaction with light.

Energy from the sun’s ultraviolet radiation causes many chemical processes on Earth. To understand, exploit, or mitigate the damage caused by these ultrafast chemical reactions, it is important to understand how they work at the atomic level.

“How do electrons and atoms interact with each other to cause certain chemical reactions?” said Oliver Gessner, a senior research scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). “This is an excellent model system for answering questions.” ).

Chemists around the world have been studying the UV photochemistry of bromoform for decades. This natural compound breaks down ozone in the Earth’s atmosphere and is naturally produced by phytoplankton and seaweed in the oceans.

The theory is that two different processes occur when exposed to UV light. In dissociation, one bromine atom separates from the rest of the molecule. Isomerization rearranges atoms into different configurations or isomers.

“Some people claim to have observed traces of this isomer, but it was too short-lived to prove it,” said Gessner, who heads the Atomic, Molecular, and Optical Sciences Program in Berkeley Lab’s Chemical Sciences Division. . Moreover, different theories predict very different proportions of bromoform following each route.

In a study published in the Journal of the American Chemical Society, Gessner and his colleagues not only confirmed this isomer formation, but also determined in what proportion of the bromoform molecules dissociated and in what proportion the isomers were formed. We developed an experiment to determine the

The researchers first excited bromoform gas molecules with ultrafast bursts of ultraviolet light (wavelength 267 nanometers), then used the relativistic ultrafast electron diffractometer at the SLAC National Accelerator Laboratory to generate ultrashort electrons. The molecules excited by the pulse were imaged. The instrument is part of the Linac Coherent Light Source at SLAC, a Department of Energy Office of Science User Facility.

“Molecules determine their direction within hundreds of femtoseconds, so we needed to go faster than that,” Gessner said.

The researchers were able to measure the distances between atoms within the bromoform molecules from electronic images and track how these distances changed over time. The analysis showed that approximately 60% of the bromoform molecules underwent isomerization within the first 200 femtoseconds of excitation and persisted for the duration of the 1.1 ps experiment.

“It was really exciting to see exactly the configuration that some people had predicted for this isomer,” Gessner said. The remaining 40% of bromoform underwent direct dissociation.

This result is an important step toward understanding bromoform photochemistry, and UV-induced photochemistry in general. “The order of chemical pathways affects the final chemical product,” Gessner said.

Benchmark measurements of the rate of isomer formation, which have long been debated, will allow us to refine the theory that predicts these reactions and their products. Furthermore, this study shows that ultrafast techniques are well-suited to give definitive answers to the questions of how fast isomers multiply and how long they survive. Gessner says this is a very powerful tool.