Surface-state transitions of Si(111)-7×7 probed using nonlinear optical spectroscopy

Surface second-harmonic generation of Si(111)-7×{}7 at RT reveals two surface-state transitions at 1.2 and 1.4 eV fundamental photon energies, in addition to the well-known two-photon resonance at 3.3 eV. A detailed analysis of the temperature-dependent spectral profile, combined with a separate sum-frequency generation and its gas-exposure measurement, attributes the 1.4-eV peak to the one-photon S₂-U₁ transition and the 1.2-eV peak to the two photon S₃-U₁ transition at 2.4 eV, which are fully consistent with the existing data on the surface states.