Alkali-Ion Desorption Energies on Polycrystalline Refractory Metals at Low Surface Coverage

Alkali-ion desorption energies have been measured under clean surface conditions as monitored by the electron work function. The average surface lifetimes of adsorbed alkali ions have been observed by pulsed desorption following diffusion through the porous metal. About 3 orders of magnitude of surface lifetimes have been investigated. For polycrystalline rhenium, tungsten, molybdenum, and tantalum, the cesium-ion desorption energies are expressed by $Q_i=0.722(\phi -I)+1.47\mathrm{}$, and for cesium, rubidium, potassium, and sodium on tungsten, by $Q_i=2.115-0.315(\phi -I)$, both in electron volts (within ±0.05 eV), with $\phi$ the work function and $I$ the ionization potential. The tungsten-alkali data are in reasonable agreement with the sum of the quantum-mechanically corrected image-charge expression and van der Waals-type interaction. If the change in surface coverage with current density is considered, the alkali-ion desorption energies measured by critical temperature for ion desorption are in tolerable agreement with the above data.