Tight-Binding-Calculation Method and Physical Origin of Reflectance Difference Spectra

The accurate tight-binding-calculation method of reflectance difference (RD) spectra is developed for surfaces and interfaces with various orientations, taking into consideration the crystalline nature and the finite size of the system adopted in the calculation. It is shown that the bond-polarization picture, i.e., that every bond becomes a unit of polarization, is applicable to the optical response of surfaces and interfaces, and the RD spectra are calculated by averaging the optical responses of even and odd finite-layer systems. The physical origins of the RD spectra are classified into two groups; the RD spectra have either the peak shape or the energy-derivative-of-peak shape depending on whether the electronic states are localized or extended around the surface/interface.