Behaviour of sodium deposited onto thin silica layers

Sodium has been deposited on silica surfaces in vacuum conditions. It has been shown by X-ray absorption spectroscopy (XAS) that its behaviour depends dramatically on the temperature of the substrate. When evaporated on clean silica surface, sodium diffuses into the bulk of the material with a well defined chemical environment which involves oxygen, sodium and silicon at distances of 2.3, 2.9 and 3.8 إ, respectively. On silica at temperature below 220 K, a metallic sodium film is formed but, upon warming the sample up to room temperature, sodium diffuses readily into the silica substrate. Finally, XAS at the silicon K-edge demonstrates that a strong modification of the mid-range order of the silicon in the silica occurs upon the dissolution of the sodium.