Renormalization of the one-dimensional π-π* Band gap on the Ge(111) 2×1 surface

Using angle resolved laser photoemission spectroscopy we have measured the transient narrowing of the surface state band gap on the cleaved Ge(111) 2×1 surface. The reconstruction of this surface results in the formation of one-dimensional π bonded chains of atoms which give rise to bonding and antibonding bands whose energy/wave vector dispersions are one-dimensional as well. Photoexcitation of this surface produces a transient π* population which, in turn, leads to an observed narrowing of the surface band gap. We have investigated this renormalization as a function of surface carrier density and laser excitation fluence.