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Dipole activity of surface phonons on Si(111)2 x 1.

We report studies of the lattice dynamics and one-phonon absorption spectrum for the ..pi..-bonded-chain model of Si(111)2 x 1. The surface conductivity is dominated by a localized mode near 50 meV associated with a longitudinal optical phonon on the surface chains which is coupled to charge fluctuations in the electronic surface states. The dipole activity of this feature is anomalously strong and, interestingly, polarized parallel to the surface, along the chain direction. The dipole activity polarized normal to the chains is also presented.