Electron-Phonon Interaction in Optical Absorption at the Si(111) 2×1 Surface

The temperature dependence of the optical absorption associated with surface states on the Si(111)2 × 1 surface is presented. The results indicate a behavior characteristic of localized excitations with a strong electron-lattice interaction, that is, a nearly Gaussian absorption that broadens roughly as the square root of the absolute temperature and shifts to lower energies upon increasing temperature. The frequency of the surface phonons that couple to the transition is smaller than hitherto assumed for the 2 × 1 surface.