Femtosecond-and-atom-resolved solvation dynamics of a Na${}^{+}$ ion in a helium nanodroplet

Abstract

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 [Albrectsen et al. Nature $\text{623}$, 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 the 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-resovled yields of the solvation complexes, Na${}^{+}$He${}_n$. For droplets containing an average of 5200 He atoms, this analysis gives a binding rate of $1.84\pm 0.09$ atoms/ps for the binding of the first five He atoms to the Na${}^{+}$ ion. Also, thanks to accurate heoretical values for the evaporation energies of the Na${}^{+}$He${}_n$ complexes, obtained by Path Integral Monte Carlo methos using a new potential energy surface presented here for the first time, we improved 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 6 ps. Measurements were carried out for three different average droplet sizes, $⟨N_D⟩=$ 9000, 5200 and 3600 helium atoms, and differences between these results are discussed.

Main text: 14 pages, 8 figures. Supplementary material: 1 page, 1 figure