Femtosecond-and-atom-resolved solvation dynamics of a Na+ ion in a helium nanodroplet

Recently, it was shown how the primary steps of solvation of a single Na+ ion, instantly created at the surface of a nanometer-sized droplet of liquid helium, can be followed at the atomic level [Albrechtsen et al., Nature 623, 319 (2023)]. This involved measuring, with femtosecond time resolution, the gradual attachment of individual He atoms to the Na+ ion as well as the energy dissipated from the local region of the ion. In this current work, we provide a more comprehensive and detailed description of the experimental findings of the solvation dynamics and present an improved Poisson-statistical analysis of the time-resolved yields of the Na+Hen ions recorded. For droplets containing an average of 5200 He atoms, this analysis gives a binding rate of 1.84 ± 0.09 atoms/ps for the binding of the first five He atoms to the Na+ ion. In addition, thanks to accurate theoretical values for the evaporation energies of the Na+Hen ions, obtained by path integral Monte Carlo methods using a new potential energy surface presented here for the first time, we improve the determination of the time-dependent removal of the solvation energy from the region around the sodium ion. We find that it follows Newton's law of cooling for the first 5 ps. Measurements were carried out for three different average droplet sizes, ⟨ND⟩ = 9000, 5200, and 3600 helium atoms, and differences between these results are discussed.