Revealing Radical Reactivity

Radical-radical reactions are notoriously difficult to track, but a twist on a standard technique has enabled researchers to study the reaction of N∙ with HO∙ to form ∙NO and H∙ (Science, DOI: 10.1126/science.1213789). The method and results should promote the study of radical reactions and better understanding of such chemistry in interstellar clouds—the regions of the universe where stars form. A group led by Kevin M. Hickson of France’s National Center for Scientific Research and the University of Bordeaux adapted a technique for low-temperature reactions that involves allowing a gas to expand from a high-pressure reservoir through a nozzle into a vacuum chamber, creating a cold, supersonic jet of gas. The researchers created N∙ from N₂ by microwave discharge just upstream of the reservoir and HO∙ by pulsed-laser photolysis of H₂O₂ directly in the supersonic flow. They then monitored the concentrations of HO∙ and the product ∙NO by laser-induced fluorescence. The team used the results to calculate rate constants for the reaction from 56 to 296 K. The results agree with theoretical predictions and should help improve chemical models of interstellar clouds, which are at 10 K, the researchers say.