Field-Emission Investigation of Thermal Desorption and Surface Diffusion of Cesium on Tungsten

Thermal desorption and surface diffusion of cesium coatings on a tungsten substrate have been investigated by pulsed-field-emission microscopy. Measured values of the heat of desorption for neutral cesium atoms vary with cesium coverage from 18 kilocalories per mole at 1.0 monolayer of cesium to 89 kilocalories per mole as the degree of coverage approaches zero. Electric field strengths up to 13 megavolts per centimeter had negligible effect, within experimental error, on the heat of neutral desorption at zero coverage. An average heat of desorption for cesium ions of 55 kilocalories per mole is obtained for the range of coverage from 0 to 0.07 monolayer, Measured values of the surface-diffusion activation energy range from 8 to 17 kilocalories per mole depending on the degree of coverage and the crystal lographic orientation of the substrate under the condition of zero electric field at the surface. Application of d-c fields of either polarity (ranging from 14 to -22 Mv/cm) is found to produce a significant decrease in the activation energy for surface diffusion. A tentative explanation of this effect is given. In the interpretation of the data, use is made of the detailed information available by examination of the field-emission-microscope patterns.