AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP22
Electron- and Photon-stimulated Desorption of Alkali Atoms from a Lunar Sample and a Model Mineral Surface@footnote 1@

Tuesday, November 5, 2002, 5:30 pm, Room Exhibit Hall B2

Session: Surface Science Poster Session
Presenter: B.V. Yakshinskiy, Rutgers, The State University of New Jersey
Authors: B.V. Yakshinskiy, Rutgers, The State University of New Jersey
T.E. Madey, Rutgers, The State University of New Jersey
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To investigate mechanisms for the origin of alkali atoms in the atmosphere of the Moon, we are studying the electron- and photon-stimulated desorption (ESD and PSD) of K atoms from a model mineral surface (SiO@sub 2@ film), and ESD and PSD of Na atoms from a piece of lunar basalt. X-ray photoelectron spectroscopy demonstrates the existence of traces of Na in the lunar sample. To permit increased signal for detailed measurements of desorption parameters (appearance thresholds, yields, energy distributions), a fractional monolayer of Na is predeposited onto the lunar sample surface. An alkali atom detector based on surface ionization and a time-of-flight technique are used for ESD/PSD measurements, together with a pulsed electron gun, and a mechanically-chopped and filtered mercury arc light source. We find that bombardment of the alkali covered surfaces by UV photons or by electrons with energy E>4 eV causes desorption of @super "@hot@super "@ alkali atoms. The results are consistent with the model developed to explain our previous measurements of sodium desorption from a silica surface@footnote 2@ and desorption of K atoms from water ice:@footnote 3@ charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. The data support the suggestion that PSD by UV solar photons is a dominant source process for alkalis in the tenuous lunar atmosphere. @FootnoteText@ @footnote 1@supported in part by NASA@footnote 2@B. V. Yakshinskiy and T. E. Madey, Surf. Sci. 451(2000) 160@footnote 3@B. V. Yakshinskiy and T. E. Madey, J. Geophys. Res. 106, E12(2001) 33303.