Abstract
By combining time-resolved two-photon photoemission (2PPE) and optical second-harmonic generation (SHG), the dynamics of surface electronic states of $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ has been studied. In the experiment the normally unoccupied part of the adatom dangling-bond bands are populated by $1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pump pulses. The 2PPE results show very fast relaxation within these bands with lifetimes ranging between 40 and $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ depending on energy relative to the Fermi level. The regime of high excitation densities was investigated by means of time-resolved SHG. With increasing pump fluence, we observed filling of the surface bands and a concomitant increase of the optical relaxation times from 0.5--$2\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The results are interpreted in terms of electron-electron scattering within a two-dimensional metallic system that is isolated from the bulk semiconductor.
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