We present a first-principles linear-response study of the phonon spectrum and dipole activity of the Si(111)2×1 surface. Our results are in good agreement with the experimental He-scattering results for the acoustic modes, as well as the electron-energy-loss data for the optical modes. We clarify a current controversy on the assignment of the electron-energy-loss peak at ∼56 meV. We find that the dominant contribution to the loss peak is anisotropic and arises from a longitudinal-optical vibration of the surface chains. However, a small but non-negligible isotropic contribution comes from subsurface modes polarized perpendicular to the surface.

• Received 8 November 1990

DOI:https://doi.org/10.1103/PhysRevB.43.8930