Field evaporation from tungsten and the bonding of surface atoms

Abstract

The field ion microscope has been used to study the sequence of removal of atoms from around the edges of (111) planes of tungsten at 77°K. A close correlation was obtained between the sequence experimentally observed in defect-free crystals and a computer simulated sequence based on a simple bonding criterion (i.e. on the total number of neighbours out as far as the 6th shell). The probability of evaporation of atoms which differ only in the number of their third nearest neighbours has been experimentally determined. Then, the differences between the activation energies for field evaporation of these atoms can be estimated if it is assumed that field evaporation is thermally activated. The results indicate that the differences in energy are not due to field enhancement and can be explained by small changes in polarizability of the atoms. On the other hand, the differences in energy are small and thermal activation energy may therefore be of less importance than tunnelling.