Ultrafast bimolecular reaction catalysis via engineered nuclear quantum effects in helium nanodroplets.

Nuclear quantum effects (NQEs) fundamentally alter chemical reactivities, particularly those occurring at low temperatures or involving light nuclei. Yet, directly resolving how NQEs influence the ultrafast dynamics of bond formation remains a central challenge. Here we report the time-resolved observation of NQEs catalyzing a light-induced bimolecular reaction within a quantum-phase environment. Using femtosecond pump-probe spectroscopy on H2-H2 and D2-D2 dimers confined in helium nanodroplets at 0.37 K, we find that the low-temperature conditions engineered by the host environment drive a pronounced acceleration of reaction dynamics. The formation of H 3 + is accelerated to 59 ± 10 fs in the droplet compared to 152 ± 17 fs in the gas phase. This catalytic effect is strongly isotope-dependent, with the heavier D 3 + exhibiting only a slight reduction in formation time from 153 ± 8 fs to 111 ± 9 fs. Molecular dynamics simulations reveal that the acceleration is catalyzed by pronounced NQEs under cryogenic confinement, with lighter hydrogen isotopes exhibiting greater quantum delocalization and consequently faster reaction rates. These findings provide a time-resolved perspective on quantum effects in ultracold chemistry, demonstrating how the unique environment of nano-cryo-reactors can harness temperature-driven NQEs to steer the ultrafast dynamics of chemical reactions.