Adsorption, Diffusion, and Evaporation of Carbon Monoxide on Tungsten

Abstract

The surface migration, adsorption, and evaporation processes for carbon monoxide on tungsten have been examined with a field-emission microscope operating at liquid helium temperature. Carbon monoxide shows three distinct temperature regions of surface behavior. The first, a low temperature spreading from the Van der Waals layer, can be observed when several carbon monoxide layers are deposited on the ``shadowed'' portion of the tungsten point. After the low temperature spread, an emission pattern identical to that of clean tungsten is obtained, so that the lattice sites are completely occupied and the bonding is uniform. The second effect occurs above about 200°K and is characterized by development of graininess of the pattern. The third region can be observed when the point is shadowed with a monolayer or less. Surface migration from this chemisorbed layer takes place above about 625°K and has associated with it an activation energy of 36 kcal/mole. Desorption occurs in a temperature region commencing at about 1000°K with an activation energy of 52 kcal/mole. The pre-exponential factor of the Fowler-Nordheim expression was useful in following the desorption process, since it is essentially a measure of the low work function areas of the total emitting surface. In this way it was found that these areas follow first-order behavior for the desorption process.