Ultrafast imaging technique reveals how ozone-damaging molecule reacts to light

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For the first time, researchers have observed how bromoform rearranges its atoms in less than a trillionth of a second after it gets hit by an ultraviolet (UV) pulse. The imaging technique captured a long-predicted pathway by which the ozone-layer-damaging molecule transforms its structure upon interaction with light.

Energy from the sun’s UV rays induces many chemical processes on Earth. To understand, use, or mitigate damage from these often ultrafast chemical reactions, it’s vital to understand how they work at the atomic level.

“How do the electrons and atoms talk to each other to make a certain chemical reaction happen? Bromoform is a prominent model system for answering these questions,” said Oliver Gessner, a senior scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

Chemists around the world have studied the UV photochemistry of bromoform for decades. The natural compound breaks down ozone in Earth’s atmosphere, and is naturally produced by phytoplankton and seaweeds in the oceans.

Theory says that it undergoes two different processes under UV exposure: in dissociation, one bromine atom breaks away from the rest of the molecule; in isomerization, the atoms rearrange into a different configuration or isomer.

“Some claim to have observed signatures of this isomer, but it was too short-lived to prove,” said Gessner, who leads the Atomic, Molecular, and Optical Sciences Program in the Chemical Sciences Division at Berkeley Lab. Moreover, different theories have widely different predictions of the proportion of bromoform that follows each pathway.

In a study published in the Journal of the American Chemical Society, Gessner and his colleagues developed an experiment that not only confirmed this isomer formation but determined what proportion of bromoform molecules undergo dissociation and what proportion form isomers.

The researchers first excited bromoform gas molecules with an ultrafast burst of UV light (267 nanometer wavelength), then imaged the excited molecules with ultrashort electron pulses using the relativistic ultrafast electron diffraction instrument at the SLAC National Accelerator Laboratory. The instrument is part of the Linac Coherent Light Source at SLAC, a Department of Energy Office of Science user facility.

“The molecules decide within a few hundred femtoseconds which way they go, so we had to be faster than that,” said Gessner.

From the electron images, the researchers could measure the distances between atoms within the bromoform molecules and track how these distances changed over time. The analysis showed that approximately 60% of bromoform molecules underwent isomerization within the first 200 femtoseconds of excitation and persisted for the duration of the 1.1-picosecond-long experiment.

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

The result is an important step toward understanding bromoform photochemistry, and UV-induced photochemistry in general. “The sequence of chemical pathways impacts the final chemical products,” said Gessner.

The benchmark measurement for a long debated isomer formation rate makes it possible to refine theories that predict these reactions and their products. Moreover, the study demonstrates that the ultrafast technique is good for providing clear-cut answers to questions about how fast isomers populate and how long they live. That, said Gessner, is a very powerful tool.