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

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