Title: The primary steps of ion solvation

Abstract:
We will present recent experimental results that have enabled us to observe the solvation dynamics of a single alkali cation ion in liquid helium with atomic resolution and on the natural femtosecond time scale.

A single Na⁺ or Li⁺ ion is created instantly at the surface of a liquid He nanodroplet and we measured in real time the gradual attachment of individual He atoms to the ion. We found that Na⁺ binds the first eight He atoms with a constant rate of one atom per 0.7 ps. This agrees with the outcome of time-dependent DFT simulations of the Na⁺ solvation process. The time-dependent yields of the Na⁺(He_n) and Li⁺(He_n) ion complexes recorded point to that solvation occurs not solely through sequential binding of He atoms but also by several He atoms attaching to the ion simultaneously. The atomic-scale information from our method opens novel possibilities for time-resolved measurements of cation-molecule complex formation.