AVS 50th International Symposium
    Surface Science Friday Sessions
       Session SS2-FrM

Paper SS2-FrM1
Desorption Induced by Electronic Transitions (DIET) from a Lunar Sample and a Model Mineral Surface*

Friday, November 7, 2003, 8:20 am, Room 328

Session: Oxide Surfaces and Interfaces
Presenter: B.V. Yakshinskiy, Rutgers University
Authors: B.V. Yakshinskiy, Rutgers University
T.E. Madey, Rutgers University
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We report recent results on an investigation of source mechanisms for the origin of alkali and alkaline earth atoms in tenuous planetary atmospheres, with focus on non-thermal processes (photon stimulated desorption (PSD), electron stimulated desorption (ESD), and ion sputtering). Whereas alkaline earth oxides (MgO, CaO) are far more abundant in lunar samples than alkali oxides (Na@sub 2@O, K@sub 2@O), the atmosphere of the Moon contains easily measurable concentrations of Na and K, while Ca and Mg are undetected there; traces of Ca have recently been seen in the Mercury's atmosphere (10@super -3@ of Na). Our experiments have included ESD, PSD and ion sputtering of alkali atoms from model mineral surface (amorphous SiO@sub 2@) and from a lunar basalt sample obtained from NASA. A comparison is made between ESD and PSD efficiency of monovalent alkalis (Na, K) and divalent alkaline earths (Ba, Ca). We find that bombardment of the alkali covered surfaces by ultraviolet photons or by low energy electrons (E > 4 eV) causes desorption of "hot" alkali atoms. This results are consistent with the model developed to explain our previous measurements of sodium and potassium desorption from a silica surface:@footnote 1,2@ electron- or photon-induced charge transfer from the substrate to the ionic adsorbate causes formation of a neutral alkali atom in a repulsive configuration, from which desorption occurs. A two-electron charge transfer to cause desorption of divalent alkaline earth atoms is a less likely process. A striking reversible temperature-dependent variation of PSD and ESD yields of alkalis from the lunar sample is found; these data have important implications for models of tenuous planetary atmospheres. The data support the suggestion that PSD by UV solar photons is a dominant source process for alkalis in the tenuous lunar atmosphere. @FootnoteText@ * supported in part by NASA@footnote 1@ BVY & TEM, Nature (1999) 400, 642@footnote 2@ BVY & TEM, Surf. Sci.(2003) 528, 54.