The enhancement by inert gases of the field desorption of oxygen from tungsten

Abstract

Field desorption of oxygen from initially field-evaporated tungsten surfaces has been investigated to determine the extent to which the adsorbate is retained on the surface under the conditions for imaging in a field-ion microscope. Desorption of oxygen occurs in vacuum for electric fields greater than half the field needed to evaporate the tungsten substrate, probably by the desorption of oxide ions. Desorption occurs to only limited extent at a particular field because the energy barrier to desorption rises as desorption proceeds. The extent of desorption is greater the higher the electric field, and at the helium imaging field little adsorbate is retained on the surface. Desorption is greatly enhanced by the presence of the inert gases used in imaging. Helium, neon and argon at 1 mtorr lower the field for desorption to a particular oxygen coverage by approximately 12, 17 and 32% respectively. Differences between the character of desorption in vacuum and in gas and estimates of the height of the energy barriers to desorption in gas strongly suggest that the enhancement of desorption is caused primarily by energetic electrons released by field ionization of the inert gas. The extent of desorption in helium and neon at 78?K indicates that very little, if any, adsorbed oxygen is retained on a tungsten surface for the periods required in recording ion micrographs without image amplification.