[Previous] | [Session 50P] | [Next]
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.)  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 Ee ~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.
 T.E. Madey, B.V. Yakshinskiy, V.N. Ageev, R.E. Johnson. J. Geophys. Res. 103, 5873-5877 (1998).