Effect of an Electric Field on the Optical Gap of Ge(111) 2 × 1 Surface States

Results of electric field modulation of internal reflections in cleaved Ge(111) 2 × 1 surfaces are presented. They can be interpreted on the basis of a buckled model of the reconstructed surface with raised (lowered) atoms negatively (positively) charged. The value of the force constant for normal displacement of surface ions (1.1 × ${10}^5$ dyn ${\mathrm{cm}}^{-1\mathrm{}}$) is estimated.