Desorption induced by electronic transitions of Na from SiO2: relevance to tenuous planetary atmospheres

Motivated by controversy concerning the origins of Na vapor in the atmospheres of Mercury and the Moon, we have studied the desorption induced by electronic transitions (DIET) of Na adsorbed on model mineral surfaces, i.e. amorphous, stoichiometric SiO2 films. We find that electron stimulated desorption (ESD) of atomic Na occurs for electron energy thresholds as low as ∼4 eV, that desorption cross-sections are high (∼1×10−19 cm2 at 11 eV), and that desorbing atoms are ‘hot’, with suprathermal velocities. Photon stimulated desorption (PSD) of atomic Na is observed to have a threshold energy of ∼4 eV, and the desorption cross-section for hν≈5 eV is ∼3×10−20 cm2. The data are interpreted in terms of charge transfer to adsorbed Na+ to form neutral Na0, which desorbs from the surface. Desorption of Na+ is observed with a threshold energy of ∼25 eV in ESD, associated with excitation of the O 2s energy level. The estimated Na desorption rate from the lunar surface via ESD by solar wind electrons is a small fraction (a few per cent) of the rate needed to sustain the Na atmosphere. However, the solar photon flux at energies ≥5 eV exceeds the solar wind electron flux by orders of magnitude; there are sufficient ultraviolet photons incident on the lunar surface to contribute substantially to the lunar Na atmosphere via PSD of Na from the surface.