Desorption of Na atoms from a lunar sample induced by photons, electrons and ions: Temperature-dependent effects

Details Desorption of Na atoms from a lunar sample induced by photons, electrons and ions: Temperature-dependent effects Desorption of Na atoms from a lunar sample induced by photons, electrons and ions: Temperature-dependent effects

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003dps....35.1506y&link_type=abstract

American Astronomical Society, DPS meeting #35, #15.06; Bulletin of the American Astronomical Society, Vol. 35, p.1483

Astronomy and Astrophysics

Astronomy

Scientific paper

We report recent results in an investigation of source mechanisms for the origin of Na atoms in tenuous atmosphere of the Moon. We are studying the photon- and electron-stimulated desorption (PSD and ESD), as well as Ar ion sputtering of Na atoms from a piece of Ti-enriched lunar basalt (#74275). 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 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]: charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. There is a strong reversible temperature-dependence of Na ESD and PSD signals, under conditions where the Na surface coverage is constant and thermal desorption is negligible. The yield of Na (atoms/photon) increases by ˜ 10X from 100K to 470K; an activation energy of ˜20 meV is measured. This phenomenon may be attributed to thermally-induced changes in surface bonding sites, for which the PSD/ESD efficiencies are different. Ion bombardment causes depletion of the Na layer both by desorption and by implantation of Na, as determined using XPS. These observations may have important implications for modeling of alkali atom concentrations in tenuous planetary atmospheres.
[1] B.V.Y. & T.E.M., Surf. Sci. 451 (2000) 160. [2] B.V.Y. & T.E.M., Surf. Sci. 528 (2003) 54.
* supported in part by NASA

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