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We present experimental results on the solvation dynamics of a single alkali cation in liquid helium, measured with atomic resolution and with femtosecond time resolution [1-2].
A single Na, K or Li atom sitting in its equilibrium position on the surface of a He nanodroplet is ionized by a 50 fs laser pulse. Thereby, an alkali ion, Ak$^+$, is introduced instantly to the liquid helium solvent from the gas phase. Hereafter, the Ak$^+$ ion will gradually pick up helium atoms to form a solvation complex, Ak$^+$He$_n$. After a time delay, a Xe atom, residing in the interior of the droplet, is ionized by a 50 fs probe pulse. The created Xe$^+$ ion pushes the Ak$^+$He$_n$ complex away from the droplet, due to the internal Coulomb repulsion. The mass and velocity of all Ak$^+$He$_n$ complexes are recorded by the combination of a Velocity Map Imaging (VMI) spectrometer and a Tpx3CAM detector.
We find that the distribution of attached helium atoms is Poissonian for the first few helium atoms. The first 3 helium atoms for Li$^+$, the first 5 atoms for Na+ and the first 11 atoms for K$^+$ all attach at a constant rate of 1.8 He/ps, in droplets containing 5200 helium atoms on average. This is in good agreement with TDDFT [1,3] and RPMD [4] simulations of the process. The time-dependent mean dissipated energy from the complexes to the droplet have also been extracted from the same measurement. Finally, a novel analysis of the detected Ak$^+$He$_n$ kinetic energies provide droplet size resolution of the above results.
[1] Albrechtsen et. al., Nature, 2023, doi:10.1038/s41586-023-06593-5
[2] Albrechtsen et. al., JCP, 2025, accepted, preprint:arXiv:2502.11783
[3] García-Alfonso et. al., JCP, 2024, doi:10.1063/5.0205951
[4] Calvo, JCP, 2024, doi:10.1063/5.0230829
