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

TL;DR

The study leverages high-resolution Penning ionization electron spectroscopy in helium nanodroplets to monitor the microhydration of NaCl, revealing a gradual transition from contact to solvent-separated ion pairs as water is added. Complementary force-field and density-functional theory calculations map the evolution of hydration structures, identifying CIP→SSIP transition around $n \approx 12$--$15$ and hydration thresholds of $n \approx 17$ for Cl$^-$ and $n \approx 34$ for Na$^+$. This integrated experimental-theoretical approach shows that full NaCl hydration, and thus complete quenching of the NaCl PIES signal, occurs near $n \approx 30$--$34$, aligning spectral features with concrete solvation shells. The work demonstrates the viability of using PIES in cryogenic helium matrices to probe solvation at the molecular level, offering insights for studying other salts and solvated complexes in a controlled environment.

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.

Figures (5)

• Figure 1: a) Penning ionization electron spectra of NaCl-doped He nanodroplets recorded at a photon energy $h\nu = 21.6$ eV. The temperature of the NaCl doping cell was $460^\circ$C and the temperature of the He nozzle is given in the figure. The cyan line shows a reference spectrum measured for an effusive beam of NaCl molecules. The peak labeled A is due to Penning ionization of NaCl molecules. The peaks B and C are due to Penning ionization of Na and He$^*$ atoms, respectively; b) peak integrals as a function of He nozzle temperature. The top horizontal scale indicates the estimated He droplet radius according to Ref. Toennies:2004.
• Figure 2: a) PIES of NaCl-doped He nanodroplets co-doped with H$_2$O molecules at variable vapor pressure in the doping chamber. The He nozzle temperature was set to 13 K, the NaCl oven temperature was $T_\mathrm{NaCl}=460^\circ$C, and the photon energy was $h\nu = 21.6$ eV. The spectrum in black was recorded in the absence of H$_2$O co-doping; b) peak integrals as a function of water co-doping pressure. The axis at the top of panel b) denotes the mean number of co-doped H$_2$O molecules per He nanodroplet, as estimated using the formulas described in SM section 2.
• Figure 3: Lowest-energy structures of selected clusters obtained from DFT calculations at small sizes, or from the force field (FF) exploration at larger sizes.
• Figure 4: Relative binding energy of various configurations of the NaCl(H$_2$O)$_n$ clusters obtained at the DFT level of theory, sorted according to the Na-Cl equilibrium distance. Each cluster size $n$ is associated with a different color, and the dashed lines join the lowest energy configurations found for all sizes.
• Figure 5: Effective coordination numbers of the Na$^+$ (upper panel) and Cl$^-$ (lower panel) ions by water molecules in low-temperature samples of NaCl(H$_2$O)$_n$ clusters with the Amber ff99 force field, keeping the distance between the two ions close to 4, 5, or 6 Å.