Rotation of Ge ad-dimers on Ge(001)

Structural calculations of the Ge ad-dimer rotation on Ge(0 0 1)-2 x I and Ge(0 0 I)-c(4 x 2) surfaces were performed by ab initio and semiempirical m ethods. Besides the two (already known) stable adsorption sites on top of t he substrate dimer rows (angles between substrate dimer bond and ad-dimer b ond are 0 degrees and 90 degrees, respectively) a new local minimum was fou nd. The angle between the substrate dimer bond and the ad-dimer of this loc al minimum is about 45 degrees. This theoretical result might explain the o bservation of the so-called A/B Ge ad-dimers on Ge(0 0 1). (C) 2001 Elsevie r Science B.V. All rights reserved.