Trihydrogen Formation on Gold Nanoparticles in Strong Laser Fields.

The trihydrogen cation (H) plays a central role in proton-transfer chemistry, astrochemical pathways, and hydrogen plasma environments, acting as a key indicator of ultrafast proton rearrangement. Although H formation has been studied extensively in the gas phase, its surface-mediated generation and its sensitivity to nanoparticle morphology remain largely unexplored. Gold nanoparticles (AuNPs), which can localize surface charge and sustain strong electric fields, offer an ideal platform to probe such nonequilibrium reaction pathways. Using reaction nanoscopy, we spatially map H production on AuNPs exposed to intense femtosecond laser fields. By comparing spherical and faceted nanoparticles, we demonstrate how morphology modulates the charge density and governs the reaction efficiency. We find that sharp features on faceted particles concentrate charge more effectively, promoting molecular fragmentation and enabling proton rearrangement and migration that enhance H yields. This work opens new directions for exploiting strong-field interactions at metal interfaces to drive nanoscale reactivity and photocatalysis.