Second-harmonic diffraction from a transient population grating of silicondangling bonds

A surface sensitive purely optical technique is demonstrated for the invest igation of rapid dephasing processes of excited electronic surface states. The combination of degenerate four-wave-mixing (DFWM) with second-harmonic generation (SHG) can be employed to measure the diffracted second-harmonic signal from a transient population grating of Si(111)7 x 7 dangling-bond st ates. By this five-wave-mixing process, which is described by a chi((4))-te nsor and is thus dipole-forbidden in bulk silicon, we deduce decoherence ti mes of similar to 5 fs.