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[50P.07] Desorption of Na from SiO₂ films, induced by low energy electrons: relevance to atmospheres of Mercury and the Moon.

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

As part of a program to probe the mechanisms by which Na atoms are produced in tenuous planetary atmospheres (e.g., Mercury, The Moon, etc.) [1] we are studying electron stimulated desorption (ESD) of sodium ions and neutral atoms from model mineral surfaces, i.e., ultrathin amorphous stoichiometric SiO₂ films (~10nm thick) grown on a metal substrate. We report new data for ESD of neutral Na, and compare with ESD of ionic Na⁺ . The neutral Na flux, desorbing from the surface under electron bombardment, is detected by using a novel surface ionization detector. This detector also permits measurements of energy distributions of neutral atoms by means of a time-of-flight method. Sodium ions, desorbing under electron bombardment, are detected by a quadrupole mass spectrometer, and ion energy distributions are measured using a retarding field method.The appearance threshold for ionic Na⁺ desorption is observed at an incident electron energy E_e ~25eV, corresponding to O2s core level excitation. A Coulomb pair O⁺ - Na⁺ is believed to form as a result of intraatomic Auger decay in the surface oxygen atom, leading to Na⁺ desorption. In contrast, the threshold for neutral Na desorption occurs for lower values of electron energy. The Na yield shows a sharp onset at E~1 to 2eV , passes through a maximum at ~8eV and rises monotonically above ~15eV (energies not corrected for work function). The desorption of neutral Na is interpreted in terms of a substrate-mediated charge transfer process. The desorbing Na atoms are "hot", with most probable kinetic energies of 0.1eV. The cross section for ESD of neutral Na by 200eV electrons is 5 \times 10^-19 cm².

[1] T.E. Madey, B.V. Yakshinskiy, V.N. Ageev, R.E. Johnson. J. Geophys. Res. 103, 5873-5877 (1998).