A long-standing puzzle regarding the Si(111) − 2 × 1 surface has been solved. The surface energy gap previously determined by photoemission on heavily n-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

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The polarization dependence of the differential reflectivity of the Si(l 11)2x1 singledomain surface has been studied experimentally. A marked anisotropy of the optical peak at 0.45 eV associated with dangling bonds is observed with maximum absorption along [Oil] directions. The chain model is in good agreement with the experimental result.