Tracking Microhydration of the NaCl Rocksalt Molecule by Quantum Chemical Calculations and Penning Ionization Electron Spectroscopy in Helium Nanodroplets

The microhydration of rock salt (NaCl) molecules was investigated theoretically by density‐functional theory and force field calculations and experimentally by high‐resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. The calculations reveal a transition from contact ion pair structures to solvent‐separated ion pairs at n=12–15. However, it takes n≈17 water molecules to form a complete solvation shell around the Cl− anion and as many as n≈34 to fully hydrate the Na+ cation, thus the entire NaCl molecule. Although NaCl molecules are predicted to be fully submerged inside the droplets, the PIES of NaCl are highly resolved, in stark contrast to other molecular species. Codoping the droplets with a controlled number of n=1–10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for n ≳ 30, in line with the calculations.