Real-time observation of the diffusion-limited formation of a cation-molecule complex.

For two molecules or atoms to react, they must first move towards each other and then, upon meeting, form new chemical bonds. Ultrafast spectroscopy and diffraction techniques have illuminated the bond-formation step for example by triggering reactions from weakly-bound precursors but not the initial approach, typically attributed to diffusion, although this is often the step that determines the reaction rate. Here, we measure and control the diffusion time for the reaction where a Li⁺ ion forms a complex with a benzene dimer, a textbook cation-π system, inside a liquid helium nanodroplet. Using femtosecond-timed Coulomb explosion, we find that Li⁺, initially at the droplet surface over 30 Å from the dimer, first solvates, then diffuses ballistically at 43 m/s and finally reacts. These results, rationalized by ring-polymer molecular dynamics simulations, pave the way for real-time imaging of stereodynamics in ion-molecule reactions.