For the first time, scientists have successfully observed how bromoform, a molecule known to contribute to ozone layer depletion, rearranges its atomic structure in less than a trillionth of a second when exposed to an ultraviolet (UV) pulse. Using a pioneering imaging technique, researchers captured a long-theorized but never-before-seen pathway by which bromoform undergoes structural transformation upon exposure to UV light. This discovery opens a window into understanding ultrafast chemical reactions at the atomic level, which are fundamental for both studying atmospheric chemistry and mitigating potential damage caused by certain chemical reactions triggered by solar energy.
The Sun’s UV rays drive numerous chemical reactions on Earth, impacting atmospheric chemistry, climate, and ecosystems. Understanding how these UV-driven processes function at the atomic level could be crucial to controlling or moderating their effects. “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,” explained Oliver Gessner, a senior scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). Bromoform has long served as a critical molecule in photochemistry research because of its ozone-depleting properties and natural presence in the environment, as it is produced by phytoplankton and seaweeds in the oceans.
Decades of studies have focused on understanding how bromoform reacts to UV light exposure. Theoretical models have suggested two primary pathways that this molecule could follow under UV light. In the dissociation pathway, one of the bromine atoms detaches from the rest of the molecule, leading to fragmentation. The second pathway, known as isomerization, involves the rearrangement of the molecule’s atoms into an alternative configuration, forming an isomer. While previous studies have hinted at the existence of this short-lived isomer, it has been difficult to directly observe and confirm its formation because of the extreme speed of these molecular changes.
In a recent study published in the Journal of the American Chemical Society, Gessner and his colleagues developed an experimental approach that not only confirmed the elusive isomer formation but also provided precise data on the proportions of bromoform molecules that follow each pathway upon UV exposure. To achieve this, the team excited bromoform gas molecules with an ultrafast UV light pulse (at a 267-nanometer wavelength) and then captured images of the molecules using ultrashort electron pulses from the relativistic ultrafast electron diffraction instrument at SLAC National Accelerator Laboratory. This state-of-the-art instrument, part of the Linac Coherent Light Source at SLAC, is capable of visualizing atomic structures at speeds fast enough to capture the movement of atoms in real-time.
The bromoform molecules, after absorbing the energy from the UV pulse, decided within a few hundred femtoseconds (one femtosecond equals one quadrillionth of a second) which path to take. Gessner emphasized the challenge: “The molecules decide within a few hundred femtoseconds which way they go, so we had to be faster than that.” By capturing electron diffraction patterns from these molecules at ultrafast speeds, the researchers were able to track atomic distances and observe how these distances evolved over time, effectively charting the pathways each molecule took.
Analysis of the data revealed that approximately 60% of bromoform molecules underwent isomerization within the first 200 femtoseconds after excitation and retained this configuration for the duration of the experiment, which lasted 1.1 picoseconds. This finding marked the first definitive observation of the isomer configuration previously predicted in theoretical models. The remaining 40% of the bromoform molecules took the dissociation route, breaking apart by losing a bromine atom.
This observation represents a major breakthrough in our understanding of bromoform photochemistry and UV-induced chemical reactions more broadly. The experiment not only confirmed the formation of the elusive isomer but also provided critical data for improving theoretical models, which can now incorporate a benchmark isomer formation rate to predict outcomes more accurately. “The sequence of chemical pathways impacts the final chemical products,” noted Gessner. Understanding this sequence allows chemists to predict and potentially control the products of photochemical reactions, which could have far-reaching implications for atmospheric science, climate modeling, and environmental protection.
Beyond expanding the understanding of bromoform, this study demonstrates the value of ultrafast electron diffraction as a precise and effective tool for capturing fleeting molecular structures. The technique offers chemists the means to address longstanding questions about the speed and lifespan of short-lived isomers, and this capability could be applied to other complex molecules in the future. “That,” said Gessner, “is a very powerful tool,” capable of answering critical questions about the ultrafast chemical processes that shape the world around us.