Tracking Microhydration of the NaCl Rocksalt Molecule in Helium Nanodroplets by Penning Ionization Electron Spectroscopy

TL;DR

This study investigates how NaCl molecules are microhydrated in helium nanodroplets using Penning ionization electron spectroscopy, revealing detailed hydration structures and the transition from contact to solvent-separated ion pairs.

Contribution

It combines high-resolution spectroscopy with theoretical calculations to elucidate the hydration process and structural transitions of NaCl in helium nanodroplets.

Findings

NaCl is highly resolved in PIES despite being fully submerged.

Quenching of NaCl ionization occurs with 5-10 water molecules, full suppression at 30+.

Complete solvation shells form around ions at specific water molecule counts.

Abstract

The microhydration of rock salt (NaCl) molecules was investigated using high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. Although model calculations predict that NaCl molecules are fully submerged inside the droplets, PIES of NaCl are highly resolved, in stark contrast to other molecular species. Co-doping the droplets with a controlled number of $n=5$--10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for $n\gtrsim 30$. Accompanying density-functional theory (DFT) and force field calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at $n=12$--15. However, it takes $n\approx 17$ water molecules to form a complete solvation shell around the Cl$^-$ anion and as many as $n\approx 34$ to fully hydrate the Na$^+$ cation, thus the entire NaCl molecule, which rationalizes the experimental findings.