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Appearance potential spectroscopy (APS): old method, but applicable to study of nano-structures.

An old surface analytical method, appearance potential spectroscopy (APS), which can probe empty electronic states, is briefly reviewed, since it has an ability to be applied to studying nanostructures. Although the theoretical calculation of the APS spectra including a two-electron process is not much simpler than that of x-ray absorption spectroscopy (XAS) spectra with a one-electron process, measurements of the APS spectra are simpler than those of the XAS ones. A careful interpretation of the spectra would lead to a characteristic and simple surface chemical analysis.