On the Theory of Monolayer Evaporation

Abstract

A theory of evaporation of a monolayer is here presented in which the evaporation of individual atoms, without interaction, is treated by statistical methods and in which the effect of interactions between atoms when the coverage is finite is treated by the thermodynamic reasoning developed by Langmuir. By combining these two approaches an expression is obtained which has, formally, only one adjustable constant, namely, the binding energy at zero coverage. The types of interactions taken into account are (following Langmuir): (a) finite size of surface atoms, and (b) dipole repulsion. Values of dipole moment are derived from thermionic emission. Assumptions have to be made regarding the partition functions relating to a surface atom. On the assumption that the only energy states available to a surface atom are those of translation parallel to the surface, agreement within the spread of currently available data for thorium on tungsten is obtained by using a binding energy at zero coverage of 7.61 ev. More precise data are required for determination of whether there must be some other and more complete specification of the partition functions for surface atoms.