Quasi-one-dimensional excitons and the optical properties of Si(111)2 x 1.

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We present theoretical evidence for strong excitonic effects at Si(111)2\ifmmode\times\else\texttimes\fi{}1. On the basis of a self-consistent calculation of electronic states, excitons, and optical properties, which involves for the first time a realistic treatment of the screened electron-hole interaction, we find a gap between surface states of 0.75 eV, and an optical spectrum in quantitative agreement with expeiment. The exciton binding energy turns out to be ${\mathit{E}}_{\mathit{b}}$=0.3 eV. Higher excitonic states are not visible, due to the quasi-one-dimensionality of the surface states