Vacuum–tunneling spectroscopy

Any spectroscopy that is to be used to study surfaces must be sensitive, almost specific to the surface. The most successful surface spectroscopies, whether they measure emission or absorption spectra of electrons, can be grouped into two general categories depending upon the origin of their surface sensitivity. The first group, which includes photoemission, Auger and appearance‐potential spectroscopy, owes its surface sensitivity to the strong electron–electron interactions of an incoming or outgoing electron, which limits the inelastic mean free path of an unscattered electron to at most a few atomic layers in the appropriate range of electron energy. The second group of electron spectroscopies, which is the subject of this article, derive their surface sensitivity from a sampling of the exponential tails of the wave functions, which tunnel into the vacuum. We have called this set of experimental techniques “vacuum‐tunneling spectroscopy.”