Chlorine onSi(001)−(2×1): Bridge versus Terminal Bonding

We present density functional calculations of the bonding structures and diffusion barriers for a Cl adatom on $\mathrm{Si}\left(001\right)-(2\times 1)$. Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and $\sim 1.1\mathrm{eV}$ 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 ( $\sim 0.6\mathrm{eV}$ 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.