Electron-, photon- and ion-stimulated desorption of Na atoms from lunar sample.

Details Electron-, photon- and ion-stimulated desorption of Na atoms from lunar sample. Electron-, photon- and ion-stimulated desorption of Na atoms from lunar sample.

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002dps....34.1904y&link_type=abstract

American Astronomical Society, DPS Meeting #34, #19.04; Bulletin of the American Astronomical Society, Vol. 34, p.876

Astronomy and Astrophysics

Astronomy

Scientific paper

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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