Science 360, 521–526 (2018)

Concentrating light in the plasmonic near-field spectrum can be useful for molecular sensing and imaging — or for boosting light-induced chemical reactions. But given the complexity of the underlying physics, the mechanism behind a certain reaction is often unclear. To understand and perhaps even control such reactions, it is important to have a detailed look at the process — if possible, at the molecular level. Yousoo Kim and co-workers now have done exactly that, by resolving light-induced bond fission in a scanning tunnelling microscope.

Forming junctions between crystalline metal surfaces and nanotips, Kim and colleagues proved that the dissociation of a surface-bound molecule occurs directly via an intramolecular absorption process. Light impinging on the tip creates plasmonic near fields that excite electrons from binding to anti-binding orbitals, which are only weakly hybridized with the metal surface states. The set-up allowed Kim and colleagues to observe bond fission in real time and at high spatial resolution, which even enabled them to rule out alternative reaction mechanisms involving charge transfer.