Electronic surface resonances of crystals

Electrons incident on a crystal surface can be temporarily trapped in surface states at energies above the vacuum level. These temporary or nonstationary surface states are observed as narrow fluctuations of elastic scattering intensity with respect to variation of electron energy and incidence direction. The scattering process is called electronic surface resonance scattering. The temporary surface states that are intermediate states in resonance scattering are called electronic surface resonances. The article surveys both experimental and theoretical research on electronic surface resonances as observed by scattering of low-energy (<1 keV) electrons. A critical account of experiments on A1(001), W(001), Ni(001), and oxygenated Ni(001) surfaces is offered together with theoretical commentary. Plots of the electronic surface resonance band structure $E\left({\mathrm{k}}_{\parallel }\right)$ ($E=\mathrm{resonance}\mathrm{energy}$, ${\mathrm{k}}_{\parallel }=\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}-\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{l}\mathrm{m}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{u}\mathrm{m}$) are compiled and the significance of $E\left({\mathrm{k}}_{\parallel }\right)$ plots for surface characterization is indicated.