Interaction of NaCl with solid water

The interaction of NaCl with solid water, deposited on tungsten at 80 K, was investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) (He I). We have studied the ionization of $Cl(3p)$ and the $1b1,$ $3a1,$ and $1b2$ bands of molecular water. The results are supplemented by first-principles density functional theory (DFT) calculations of the electronic structure of solvated $Cl−$ ions. We have prepared NaCl/water interfaces at 80 K, NaCl layers on thin films of solid water, and $H2O$ ad-layers on thin NaCl films; they were annealed between 80 and 300 K. At 80 K, closed layers of NaCl on $H2O,$ and vice versa, are obtained; no interpenetration of the two components $H2O$ and NaCl was observed. However, ionic dissociation of NaCl takes place when $H2O$ and NaCl are in direct contact. Above 115 K solvation of the ionic species $Cl−$ becomes significant. Our results are compatible with a transition of $Cl−$ species from an interface site (Cl in direct contact with the NaCl lattice) to an energetically favored configuration, where Cl species are solvated. The DFT calculations show that $Cl−$ species, surrounded by their solvation shell, are nevertheless by some extent accessed by MIES because the $Cl(3p)$-charge cloud extends through the solvation shell. Water desorption is noticeable around 145 K, but is not complete before 170 K, about 15 K higher than for pure solid water. Above 150 K the NaCl-induced modification of the water network gives rise to gas phase like structures in the water spectra. In particular, the $3a1$ emission turns into a well-defined peak. This suggests that under these conditions water molecules interact mainly with $Cl−$ rather than among themselves. Above 170 K only Cl is detected on the surface and desorbs around 450 K.