Temperature-dependent DIET of alkalis from SiO2 films: Comparison with a lunar sample

We present recent results in an investigation of source mechanisms for the origin of alkali atoms (Na, K) in tenuous planetary atmospheres. A reversible temperature dependence has recently been observed in the electron and photon stimulated desorption (ESD and PSD) of Na from a lunar basalt sample. The observations were attributed to a temperature-related variations in binding sites with different desorption rates. We have now measured the reversible temperature-dependence of the ESD yields for neutral Na and K, and ionic Na+ and K+ from an SiO2 surface. The neutral desorption yields demonstrate opposite behavior from the lunar sample, which is presumably associated with different desorption mechanisms. The sticking probability S for atomic K is nearly constant over the substrate temperature range 100-500 K, whereas S for Na decreases with increasing T in this range. To clarify the charge-transfer desorption mechanism, we compare the DIET of monovalent atoms (Na, K) and divalent atoms (Ba). The threshold for ESD of Ba is ∼25 eV, much higher than that for Na, K (3 and 4 eV).