Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation

We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52-60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.