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

Motivated by controversy concerning the origins of Na vapor in the atmosphe res of Mercury and the Moon, we have studied the desorption induced by elec tronic transitions (DIET) of Na adsorbed on model mineral surfaces, i.e. am orphous, stoichiometric SiO2 films. We find that electron stimulated desorp tion (ESD) of atomic Na occurs for electron energy thresholds as low as sim ilar to 4 eV, that desorption cross-sections are high (similar to 1 x 10(-1 9) cm(2) at 11 eV), and that desorbing atoms are 'hot', with suprathermal v elocities. Photon stimulated desorption (PSD) of atomic Na is observed to h ave a threshold energy of similar to 4 eV, and the desorption cross-section for hv approximate to 5 eV is similar to 3 x 10(-20) cm(2). The data are i nterpreted in terms of charge transfer to adsorbed Na+ to form neutral Na-0 , which desorbs from the surface. Desorption of Na+ is observed with a thre shold energy of similar to 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 a t energies greater than or equal to 5 eV exceeds the solar wind electron fl ux by orders of magnitude; there are sufficient ultraviolet photons inciden t on the lunar surface to contribute substantially to the lunar Na atmosphe re via PSD of Na from the surface. (C) 2000 Elsevier Science B.V. All right s reserved.