Behaviour of sodium deposited onto thin silica layers

Sodium has been deposited on silica surfaces in vacuum conditions. It has b een 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 d istances of 2.3. 2.9 and 3.8 Angstrom., respectively. On silica at temperat ure below 220 K, a metallic sodium film is formed but, upon warming the sam ple up to room temperature, sodium diffuses readily into the silica substra te. Finally. XAS at the silicon K-edge demonstrates that a strong modificat ion of the mid-range order of the silicon in the silica occurs upon the dis solution of the sodium. (C) 2001 Elsevier Science B.V. All rights reserved.