Single-atom point source for electrons: Field-emission resonance tunneling in scanning tunneling microscopy

Many years ago spectroscopic evidence based on a single-electron process, namely, field-emission resonance tunneling, was reported showing the electronic structure of single atoms adsorbed on metal surfaces. Huge enhancements in the highly collimated tunneling current through the virtual states of the adparticle made the single-atom spectroscopy feasible when the field-emission spectrometer was operated in the ``probe-hole mode.'' A related effect is currently popular in today's scanning tunneling microscopy and also in nanometer-cluster spectroscopy. The phenomenology and theory of the effect is presented and the current activity is considered in light of what has been uncovered in the field-emission work.