Mean residence time of potassium ions on tungsten as measured with reference to ionization efficiency

The mean residence time, τ_i, of potassium ions on “clean” and oxygenated tungsten has been measured, together with the ionization efficiency, as a function of surface temperature T by using incident K beams of low intensity (10⁹–10¹² atoms cm^?2 s^?1). For T higher than ~900 K the observed τ_i followed Frenkel's equation $\text{τ}{}_{\mathrm{<mtext>i</mtext>}}\text{= τ}{}_{\mathrm{<mtext>i</mtext>}}{}^0\text{exp}\text{(}\text{Q}{}_{\mathrm{<mtext>i</mtext>}}\text{kT}\text{)}$ as usual and the agreement of the ionic desorption energy Q_i and of the pre-exponential factor τ_i⁰ with the corresponding values of previous experiments was quite satisfactory. Below 830 ~ 910 K, where a steep drop of ionization efficiency began to be noticeable, Arrhenius plots of τ_i deviated considerably from linearity. The apparent increment of the desorption energy was shown to be nearly equal to the decrement of thermionic work function of tungsten as obtained from the ionization efficiency and Saha-Langmuir equation. The increase of surface coverage by potassium was accordingly taken as the main cause of the departure of Arrhenius plots from linearity. Under certain conditions of incident beam intensity and surface temperature τ_i was observed to make an abrupt change from a higher to a lower value — a difference expressed as 100–140 meV in terms of the difference in ionic desorption energy. This peculiar phenomenon was attributed to the phase change of adsorbed potassium on tungsten.