Time-resolved solvation dynamics of Li${}^{+}$, Na${}^{+}$ and K${}^{+}$ ions in liquid helium nanodroplets

Abstract

In 2023, ultrafast pump-probe spectrocopy was used to record the solvation dynamics of a single Na${}^{+}$ ion in a liquid helium droplet, atom-by-atom and with femtosecond time resolution [Albrechtsen \textit{et al., Nature}, 2023, \textbf{623}, 319]. Subsequently, theoretical studies showed that other alkali ions solvate in a similar manner but no experimental results were reported so far. Here, we extend the previous measurement on Na${}^{+}$ to Li${}^{+}$ and K${}^{+}$ ions. A pump pulse selectively ionizes an alkali atom, initially residing at the droplet surface, and the ensuing solvation dynamics of the formed alkali cation, Ak${}^{+}$, is followed by ionizing a Xe atom, located in the droplet interior, and recording the yields of Ak${}^{+}$He${}_n$ ions expelled from the droplet as a function of the pump-probe pulse delay. We find that Li${}^{+}$, Na${}^{+}$ and K${}^{+}$ ions solvate with a binding rate of 1.8 $\pm$ 0.1, 1.8 $\pm$ 0.1 and 1.7 $\pm$ 0.1 He per ps, respectively. Furthermore, by comparing the number distribution of the Ak${}^{+}$He${}_n$ ion yields to the evaporation energies of these ion--He complexes, obtained by Path Integral Monte Carlo calculations, we identify signatures of the first solvation shells of Li${}^{+}$, Na${}^{+}$ and K${}^{+}$. Lastly, we determine the time-dependent dissipation of the solvation energy from the vicinity of the three alkali ion species and find that the rate is highest (lowest) for Li${}^{+}$ (K${}^{+}$)