Optical reflectivity of the Si(111)-(2x1) surface - The role of the electron-hole interaction

We calculate the optical reflectivity of the Si(111)-(2 x 1) surface from f irst principles. To this end, we first calculate the quasiparticle band str ucture of the surface within the CW approximation for the electronic self e nergy. The band structure exhibits two surface bands inside the fundamental bulk band gap. Thereafter the electron-hole interaction is computed for tr ansitions between the relevant bands, the Bethe-Salpeter equation for coupl ed electron-hole excitations is solved and the optical response is evaluate d. In the energy range below the fundamental bulk band gap, the reflectivit y spectrum is dominated by a surface exciton at 0.43 eV with an excitonic b inding energy of 0.26 eV. Our calculated spectrum is in very good agreement with experimental data from differential reflectivity spectroscopy.