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B. V. Yakshinskiy, T. E. Madey (Rutgers University)
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 Na atoms from a piece of Ti-enriched lunar basalt (#74275, 200); we also measure ion-induced desorption (sputtering by He@super +@, Ar@super +@). X-ray photoelectron spectroscopy (XPS) 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 “hot” alkali atoms. The results are consistent with the model developed to explain our previous measurements of sodium and potassium desorption from a silica surface [1, 2] and desorption of K atoms from water ice [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 desorption yield dramatically depends on the sample temperature. The data support the suggestion that PSD by UV solar photons is a dominant source process for alkalis in the tenuous lunar atmosphere. Ion bombardment causes depletion of the Na layer both by desorption and by implantation of Na, as determined using XPS.
[1] B.V.Y. & T.E.M., Surf. Sci. 451(2000) 160 [2] B.V.Y. & T.E.M., Surf. Sci. (2002), submitted. [3] B.V.Y. & T.E.M., J. Geophys. Res. 106, E12 (2001) 33303
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.