‘Roaming’ molecular fragments captured in real time

Sometimes atoms, like pets and adventuresome hikers, slip loose and wander off into the wild. Their final destination isn’t known, and their trajectory can be all over the map. It’s not so easy to track their path.

But an international research team that includes Paul Houston, the Peter J.W. Debye Professor Emeritus of chemistry and chemical biology in the College of Arts and Sciences, has done just that: They’ve made the first direct observation, in real time, of an elusive phenomenon – “roaming” reactions, in which a chemical compound breaks apart and its molecular fragments drift chaotically in orbit before re-forming into new compounds.

The team’s findings were published Nov. 27 in Science.

The project was led by research associate Heide Ibrahim at the Institut National de la Recherche Scientifique (INRS) in Quebec, working in collaboration with researchers at the INRS’s Advanced Laser Light Source; Nagoya University in Japan; the National Research Council in Ottawa; and Emory University.

The work builds off recent research that showed in unprecedented detail what happens when roaming reactions – initiated by sunlight – occur in the atmosphere.

Like the previous paper, the researchers focused on formaldehyde (H2CO) and used a process known as photo-dissociation. Strong, ultrashort laser pulses were fired at a H2CO molecule, exploding it and setting hydrogen fragments loose. As those fragments roamed in orbit around fragments of the formyl radical HCO, the researchers took snapshots at various checkpoints along the route.

But they had to be quick about it. The experimental signal was 10 billion times shorter than a millisecond. They used a technique called Coulomb explosion imaging (CEI) that uses ultrashort laser pulses, in combination with statistical analysis, to reconstruct the momentum of the fragments.

Houston provided theoretical support for the research.

“It was a great pleasure for me to be able to support the work of Dr. Ibrahim and her colleagues with calculations that they could compare to their results, and use to improve the apparatus,” Houston said. “Their experimental technique allowed them to observe rare and interesting results against a near overwhelming background of other events, all on an ultrafast time scale. The results give us the hope that we can develop even more accurate methods to watch molecules move and ultimately to understand the causes for their fascinating chemical transformations.”

The pathway of roaming fragments had been previously observed in theoretical simulations, but was only inferred in experiments. Now that roaming has been captured in real time, it could shed light on a host of other chemical reactions.

“Although roaming remains an elusive process that is difficult to grasp, this scientific breakthrough provides insight into how to measure it – as well as other statistical processes that require highly sensitive detection, in the face of highly disruptive background signals,” Ibrahim said. “Ultimately, this may be just the beginning of another winding journey towards some of Mother Nature’s secrets. Roaming is a process whose role in environmental and atmospheric chemistry is only at the beginning of being understood.”

The research was supported by the Canada Foundation for Innovation; the Natural Sciences and Engineering Research Council of Canada; Fonds de Recherche du Québec; the Japan Society for the Promotion of Science; and Nagoya University.

Read the story in the Cornell Chronicle.

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