Core-Level Binding Energy Shifts in Ultrathin Alkali-Halide Films on Metals: KCl on Ag(100)

We present an experimental and theoretical analysis of the core-level binding energy shifts in metal-supported ultrathin potassium chloride (KCl) films, i.e., a case from a broader class of few-atom-thick, wide-band gap insulating layers that is increasingly used in nanosciences and nanotechnologies. Using synchrotron-based high-resolution photoemission spectroscopy (HRPES) measurements, we identify the different contributions to the core-level binding energy shifts for the Cl- anions and K(+ )cations of two to three atomic layer-thick KCl films grown on Ag(100). The distances of the Cl- and K+ ions of the first two atomic layers of the KCl film from the metal substrate are determined from normal incidence X-ray standing wave measurements. We also calculated the core-level binding energy shifts using an analytical electrostatic model and found that the theoretical results are in agreement with the experimental HRPES results only when polarization and substrate-induced image charge effects are taken into account. Finally, our results evidence the effect of the third atomic layer of the KCl film, which partially covers and screens the first two atomic layers of KCl, wetting the metal substrate.