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

This article focuses on the femtosecond time-resolved study of the solvation dynamics of a single sodium ion (Na⁺) in helium nanodroplets. Using pump-probe laser ionization and velocity map imaging, the researchers measured the sequential attachment of helium atoms to Na⁺ and analyzed the resulting Na⁺Heₙ ion yields with a Poisson statistical model, determining helium binding rates that increase with droplet size (1.65–2.04 atoms/ps for droplets averaging 3600 to 9000 He atoms). Complementary path integral Monte Carlo calculations provided evaporation energies for Na⁺Heₙ complexes, enabling the estimation of time-dependent energy dissipation from the ion's local environment, which was found to follow Newton's law of cooling within the first 5 picoseconds. The study also includes molecular dynamics simulations supporting assumptions about helium evaporation dynamics post-ejection, contributing to a detailed understanding of ion solvation and energy transfer processes in superfluid helium droplets.