Nonthermally Accessible Phase for CO on the Si(100) Surface

A new phase $\mathrm{BT}-\mathrm{C}\mathrm{O}$ on Si(100), not accessible with thermal CO (which leads to the $T-\mathrm{C}\mathrm{O}$ phase), has been observed using an energetic molecular beam of CO and characterized by first principles quantum chemical methods. The $\mathrm{BT}$ phase is composed of bridge $\left(B\right)$ and terminal $\left(T\right)$ sites; it is driven and stabilized by cooperative electrostatic interactions that result because $B$ $\left(T\right)$ binding leads to the transfer of charge away from (toward) the surface. Desorption of CO from the $B$ site proceeds through the $T$ configuration.