Chemical bond formation showing a transition from physisorption to chemisorption

Surface molecules can transition from physisorption through weak van-der-Waals forces to a strongly bound chemisorption state by overcoming an energy barrier. We show that a CO molecule adsorbed to the tip of an atomic force microscope enables a controlled observation of bond formation, including its potential transition from physisorption to chemisorption. During imaging of Cu and Fe adatoms on a Cu(111) surface, the CO was not chemically inert but transited through a physisorbed local energy minimum into a chemisorbed global minimum, and an energy barrier was seen for the Fe adatom. Density functional theory reveals that the transition occurs through a hybridization of the electronic states of the CO molecule mainly with s-, pz- and dz2-type states of the Fe and Cu adatoms, leading to chemical bonding.