Photon-stimulated desorption of Na from a lunar sample: temperature-dependent effects

We report recent results in an investigation of source mechanisms for the origin of Na atoms in tenuous planetary atmospheres, focusing on non-thermal processes. Experiments include photon stimulated desorption (PSD), electron stimulated desorption (ESD), and ion sputtering of Na atoms from the surface of a lunar basalt sample. Bombardment of the sodium covered surface by 3 keV Ar+ ions removes Na from the surface by sputtering into vacuum and by implantation into the sample bulk. Bombardment of the Na covered surface by ultraviolet photons or by low energy electrons (E>3 to 4 eV) causes desorption of “hot” Na atoms. These results are consistent with our previous measurements of sodium and potassium desorption from a silica surface: electron- or photon-induced charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. There is a strong temperature-dependence of Na ESD and PSD signals, under conditions where the Na surface coverage is constant and thermal desorption is negligible. The yield of Na (atoms/photon) increases by 10× from 100 to 470 K; an activation energy of ∼20 meV is measured. This phenomenon may be attributed to thermally-induced changes in surface bonding sites, and will affect recent modeling of the sodium atmospheres of Mercury and the Moon.