Study of the mechanism of thermal positive ion production on a heated metal surface and development of a themal positive ion source

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The ionization efficiency (<beta>^+) for producing Li^+ ions from LiI molecules incident upon a rhenium surface heated in a thigh vacuum (2 x 10^<-7> Torr) was measured as a function of surface temperature (T<similar or equal>1000 - 2000 K). The data thus obtained were analyzed by our original theory on dissociative positive surface ionization, thereby indicating that (1) <beta>^+ becomes unity at T<similar or equal>1200 - 1400 K because the work function (0^+) of the surface increases from about 5.5 eV up to 6.5 eV owing to adsorption by residual gas molecules (RGM), (2) <beta>^+<similar or equal> 0.1 at T<greater than or similar>1700 K where 0^+<similar or equal>5.5 eV, corresponding to an essentially clean surface, and (3) <beta>^+ begins to decrease sharply at R<similar or equal>1200 K because of co-adsorption of LiI and RGM. When Li atoms were impinged upon the surface, on the other hand, <beta>^+ and 0^+ were nearly constant at about 0.08 and 5.4 eV, respectively, at R<greater than or similar>1200 K, giving evide evidence that adsorption fo Li atoms suppresses the work function increase due to adsorption of RGM. Mass spectrometric analysis of RGM and also introduction of air up to 2 x 10^<-5> Torr into the ion source region gave the additional evidence that the work function increase found in the middle temperature region is caused mainly by adsorption of xygen, thereby promoting surface ionization of the sample molecules, just as predicted by our theory. By using a new technique developed by us, the mean adsorption lifetime of NaI, LiI, LiBr and LiCl was determined together with heat of adsorption and entropy change due to adsorption of each sample molecule. On the basis of these results, a small ion source was developed, by which alkali ions of more than 10 nA were readily produced from alkali halide molecules.