Abstract. This paper begins with a brief survey of the literature dealing with the adsorption and desorption of alkalis on oxide surfaces. Emphasis is on desorption phenomena: thermal desorption, electron-and photon-stimulated desorption, and ioninduced desorption (sputtering). Then the relevance of these data to the desorption of alkalis from mineral surfaces and to the origins of alkali vapors in tenuous planetary atmospheres is discussed. The data presented for Na and K indicate that desorption processes initiated by thermal or electronic excitations do not depend strongly on whether the Na returns to the surface or diffuses up through the regolith, and that neutral yields dominate ion yields in all cases. Although the desorbed neutral energy distributions are not well approximated by Maxwell-Boltzmann distributions, the mean energies of the desorbed neutral Na and K are seen to be consistent with the temperatures extracted for the "hot" component of the lunar atmosphere. This suggests that the "hot" component may be produced by electronically stimulated desorption (e.g., electron-stimulated desorption and/or photon-stimulated desorption). If this is the case, a possible "size effect" may be operative, in which desorbed neutral K atoms are somewhat more energetic than desorbed Na. In such desorption processes a low-energy component may be generated by scattering of desorbing atoms in the porous regolith; thermal desorption can also generate low-energy atoms. The data further indicate that thermal desorption should be rapid in the equatorial regions of Mercury, possibly depleting this region of alkalis, whereas thermal desorption should be less efficient on the Moon. Surface charging may be important at the surface of the Moon, by accelerating the solar electrons to energies above the threshold for' initiating alkali desorption. Suggestions are made for future laboratory work.

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The adsorption and thermal desorption of Na from thin SiO2 films have been studied. X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), low energy ion scattering (LEIS), temperature-programmed desorption (TPD), low energy electron diffraction (LEED) and work function measurements have been used to characterize the growth mechanism and properties of stoichiometric SiO2 films deposited onto a Re (0001) substrate. Upon deposition of Na onto SiO2 at 250 K, the first monolayer of Na exhibits ionic character, and evidence of metallic Na (plasmon features in XPS) is observed for higher coverages. TPD spectra for Na from SiO2 include a monolayer peak at ~700 K, and the multilayer peak due to sublimation of bulk Na at ~330 K. Penetration of Na into SiO2 can be induced by heating, or by He ion bombardment of a Na/SiO2 layer. The sticking probability for Na on SiO2 is ~0.5 at 250 K, and it decreases at higher substrate temperatures.