Desorption of Na from Model Mineral Surfaces: Relevance to Origins of Na in the Atmospheres of Mercury and the Moon

T. E. Madey, B. V. Yakshinskiy (Rutgers University)

In an effort to probe mechanisms for the origin of Na in the atmospheres of the planet Mercury and the Moon, we are studying the adsorption and desorption of Na deposited on model mineral surfaces, amorphous SiO2 thin films. Amorphous stoichiometric silica thin films have been grown on a Re(0001) surface at 300K by silicon evaporation in an oxygen ambient, followed by high temperature annealing. The adsorption-desorption of Na deposited onto these films have been investigated using x-ray photoelectron spectroscopy, low energy ion scattering, thermal desorption spectroscopy, and electron stimulated desorption (XPS, LEIS, TDS, and ESD). Na is found to grow on amorphous SiO2 as a mostly ionic monolayer, followed by metallic island formation (Stranski-Krastanov growth mode); there appears to be little diffusion of Na to the sample bulk at room temperature. Fractional monolayers of Na desorb thermally in the temperature range 500-1000K, with desorption of multilayer Na at 350K. Na desorption is affected by ion bombardment, and by reaction with residual gases. ESD of Na+ ions is observed for incident electron energies above a threshold of 25 eV, corresponding to the O2s excitation. The data are discussed in the context of recent data for photon-stimulated desorption of alkalis from oxide surfaces.