Resonant surface-state transitions of Si(111)-7×7 measured with two-photon photoemission spectroscopy
Two-photon photoemission (2PPE) spectroscopy of the Si(111)-7×7 surface reveals resonant photoexcitation between surface states as well as between bulk bands. By finely tuning the photon energy from 3.6 to 4.5 eV, three series of structures A, B, and C, are identified in the 2PPE spectra. Peak A arises from an occupied state at -0.62 eV from EF, and is enhanced by resonance with an intermediate state at 3.45 eV. The states are attributed to the occupied and the unoccupied bulk bands. Peak B occurs from a transition between an occupied surface state at -1.88 eV and an unoccupied surface state at 1.94 eV. These states are assigned to the bonding and antibonding states of the Si-Si bond beneath the adatom. Structure C arises from an unoccupied surface state at 0.79 eV. The polarization dependence of the spectra supports these assignments. These results are consistent with energies of the known bulk band structure, and the occupied/unoccupied surface states. Characteristics of the 2PPE results are (i) the structures are clearly observed only within a limited photon energy region of approximately 0.3 eV width and (ii) no transition between a surface state and a bulk state has been observed.