The solvation of ions is a fundamental process in chemical and biological systems. The phenomenon is well-understood at the macroscopic level, but remains more elusive at the atomic scale due to several issues: How do you define when the solvation process starts? How do you measure individual atoms/molecules as they bind to the solute? How do you stop the solvation process once it has begun?
This project aims to address these issues by combining liquid helium droplets, femtosecond laser pulses and velocity-map imaging spectroscopy. The fundamental idea is the following: a nanometer-sized (3~5 nm) droplet of liquid helium is doped with a single alkali metal atom and a single xenon atom. The alkali metal atom resides at the surface of the droplet in an unsolvated state, while the xenon atom is fully submerged into the center of the droplet. An ultrafast pump laser pulse (~35 x 10-15 s long) is used to selectively ionize the alkali atom. The ion will subsequently solvate into the droplet by attaching individual helium atoms. A second, ultrafast probe laser pulse (~50 x 10-15 s) ionizes the xenon atom in the center of the droplet. The two positive charges repel, which forces the alkali ion out of the droplet along with any helium atoms attached. By varying when the probe laser pulse arrives, we can therefore control the evolution of the solvation process, atom-by-atom with femtoseccond time-resolution.
We have established that the solvation dynamics of Li+, Na+ and K+ ions can be measured using this method. The results show that the ions solvate with roughly the same solvation rate up until a specific solvation shell, which varies based on the size of the ion. The current goal of this project is to extend the method to the alkaline-earth (AkE) metals (Mg, Ca, Sr, Ba), which have two ionic states (AkE+ and AkE2+) accesible by the pump laser pulse. Realizing this goal requires building a new doping oven, capable of reaching 1200 °C with vacuum-proof water cooling, and assembling optics for a resonant-enhanced multiphoton ionization (REMPI) laser scheme.
Relevant publications:
Albrechtsen, S.H. et al. Observing the primary steps of ion solvation in helium droplets. Nature 623, 319–323 (2023).
Albrechtsen, S.H. et al. Femtosecond-and-atom-resolved solvation dynamics of a Na+ ion in a helium nanodroplet. J. Chem. Phys. 7 May 2025; 162 (17): 174309.
Christensen, J. K. et al. Time-resolved solvation dynamics of Li+, Na+ and K+ ions in liquid helium nanodroplets. Phys. Chem. Chem. Phys., 2025, 27, 24184-24194
García-Alfonso, E. et al. Time-resolved solvation of alkali ions in superfluid helium nanodroplets: Theoretical simulation of a pump–probe study. J. Chem. Phys. 14 October 2025; 163 (14): 144309.
Calvo, F. Concurrent processes in the time-resolved solvation and Coulomb ejection of sodium ions in helium nanodroplets. J. Chem. Phys. 28 September 2024; 161 (12): 121101.
