Determining the evaporation energies of alkali and alkaline earth metal atoms using field desorption

Abstract

The desorption of cesium and barium atoms from a quasi-spherical nanostructured surface of a field emitter has been studied using the method of field desorption microscopy. The dependences of the desorbing electric field strength on the degree of cesium and barium coating of the rhenium and tungsten field emitter at migration equilibrium are obtained. Migration equilibrium occurs at a certain emitter temperature when, due to the surface diffusion, the concentration of adsorbate is redistributed on various surface areas depending on the local heat of evaporation of adatoms from these areas. It is shown that when the migration equilibrium condition is met on the nanostructured surface of the field emitter, on which there are flat low-index crystal faces with different output work, the desorption has an avalanche-like character with the removal of the entire adsorbate. This behavior of the field desorption is characteristic of atoms of alkaline and alkaline earth metals, associated with a sharper increase in the work function compared with an increase in the evaporation energy of the atom with a decrease in the adsorbate concentration. As a result, the ion desorption energy decreases with an avalanche-like increase in the desorption rate. Taking into account the same desorbing field on all parts of the surface, which follows from the characteristics of Fowler Nordheim, the heat of evaporation of adsorbate atoms on different parts of the surface with different work function is determined within the framework of the image forces model for field desorption.

Keywords

• field desorption microscopy
• atom evaporation energy
• field emitter
• rhenium
• tungsten
• alkaline and alkaline earth metals
• migration equilibrium