Desorption and Migration of Cesium Adsorbed on Tungsten in the Presense of an Electric Field

Studies of the field desorption of cesium from tungsten and the equilibrium concentrations of cesium under electric fields are made by a field emission microscope (FEM). It is observed that, at small concentrations, the desorption begins from (211) vicinities and proceeds along cross-bone zones, while, at large concentrations, the desorption occurs instantaneously from the whole surface of the tip apex. The threshold field strength for desorption increases with the increase of the surface concentration of cesium. Surface migration of Cs atoms towards the tip apex from the tip shank following the field desorption is suppressed by an applied negative electric field, and the equilibrium concentration between the apex and the shank appears. These equilibrium concentrations under various electric fields are determined, and the results are used to calculate the dipole moments of the ad-atoms for many concentrations.