Ultrafast-electron dynamics and recombination on the Ge(111)(2×1) π-bonded surface

Angle-resolved laser-photoemission spectroscopy has been used to study the ultrafast-electron scattering and recombination processes on the Ge(111) π-bonded (2×1) surface with subpicosecond time resolution. Electrons photoexcited into the bulk Ge conduction band scatter into the unoccupied surface antibonding ${\mathrm{\pi }}^{\mathrm{*}}$ band whose minimum is at the J¯ point in the surface Brillouin zone. Rapid relaxation to the surface-band minimum is followed by a unique phonon-assisted process in which electrons recombine with bulk holes at the valence-band maximum, which we find to be the primary mechanism responsible for the decay of the transient ${\mathrm{\pi }}^{\mathrm{*}}$ population. Time-dependent measurements carried out at 300 and 120 K have been employed to determine the role of energetic phonons in the scattering processes. These processes are modeled with a set of rate equations, whose fits to the data yield scattering times used to determine a surface recombination velocity directly. Ultrafast surface-state hole dynamics are observed, and a renormalization of the surface band gap is studied as a function of electron density. The π-bonded states are fundamentally one dimensional in nature, and thus these results represent studies of band-gap renormalization in a one-dimensional system.