Atoms and molecules, the invisible building blocks of everything around us, quickly become complicated as their size and numbers increase. A molecule of just two atoms can occupy any of a multitude of rotational, vibrational, and electronic quantum states, each with potentially different behavior in a chemical reaction. With lasers and molecular beams, physical chemists can prepare and probe many of those states individually and thus dissect the dynamics of a gas-phase molecular reaction in exquisite detail.

But when a reaction takes place on a solid surface—a common scenario in industrial catalysis, materials science, and geology—it’s much more of a black box. Because every atom on a rough, irregular surface is situated a little bit differently, they can have dramatically different reactivities, even to the point of steering the reaction toward different sets of products (see Physics Today, September 2018, page 17).

Those chemical distinctions among surface sites...

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