On the Adsorption of Oxygen on Tungsten as Revealed in the Field Emission Electron Microscope

The tungsten surface in a field emission microscope is exposed to oxygen gas at a pressure near 5×10—8 mm and the effect of the gas on the field emission is observed. During the first 30 seconds, the emission decreases rapidly, but some regions deactivate more rapidly than others. During the next 12 minutes, the rate of deactivation is much slower, but again the change in activity for some planes is much more pronounced than for others. After the surface is heavily covered, the oxygen is desorbed by glowing the tungsten for one minute at successively higher temperatures. The chief conclusions are: Oxygen is adsorbed on tungsten in three stages or layers. In the first layer, the bond strength is about 5 electron volts and the oxygen cannot be removed below about 1500°K. In the second layer, the bond strength is about 2.5 ev and the oxygen cannot be removed below about 750°K. These bond strengths depend somewhat on the crystallographic plane. In the third layer, the oxygen forms a variety of molecular complexes with the substrate. These reveal themselves as intensely bright, highly localized groups of spots. Each spot is due to electrons emitted by individual atoms in a molecule. The conditions necessary to observe such adatoms are given.