Native oxidation of the $\mathrm{S}\mathrm{i}(111)(1\times 1)\mathrm{H}$ surface causes the appearance and disappearance of second-harmonic generation (SHG) resonances related to specific bonding configurations of Si atoms at the interface. Resonances at 3.52 eV two-photon energy observed in $p$-polarized SHG spectra are indicative of a Si suboxide configuration present in a partially oxidized Si surface bilayer. Similar resonances are observed in spectra of thermally oxidized Si(111) and point to ${\mathrm{S}\mathrm{i}}^{2+}$ suboxide states at the buried interface.

• Received 5 September 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.097402