Chlorine on Si(001)-(2x1): Bridge versus terminal bonding

We present density functional calculations of the, bonding structures and diffusion barriers for a Cl adatom on Si(001)-(2 x 1). Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and similar to 1.1 eV higher in energy than the DB is found. The calculated properties of this state agree with recent ESDIAD and HREELS observations. This bridge-bonded site is not along the Cl intradimer diffusion pathway of lowest energy. For this path a transition state also having a bridging structure (but not breaking the dimer bond) and rather low in energy (similar to 0.6 eV with respect to the DB) is determined. The low intradimer barrier is consistent with the facile switching of Cl recently observed in scanning tunneling microscopy experiments.