Potassium adsorption on the polar ZrC(111) surface: photoemission spectroscopy study

Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study an adsorption mechanism of K on a ZrC(111) surface. Coverage-dependent measurements of the work function and the K 3p core-level photoemission reveal that the adsorbed K atoms are partially ionized in the initial stages of adsorption, and that the metallic overlayer is formed at high coverages through a continuous depolarization of the overlayer with increasing coverage. Valence-band photoemission measurements show that the Zr 4d -derived surface state on ZrC(III) is hybridized with the 4s states of the adsorbed K atoms to form occupied bonding and unoccupied antibonding states at low coverages. This, together with the fact that adsorbed K is ionized in the initial stages, suggests that the bonding state should be mainly composed of the Zr 4d orbitals, while the antibonding state has a dominant contribution of the K 4s orbitals. As the coverage increases, the antibonding state shifts downwards and becomes an occupied state around the Γ̄ point in the high coverage region where the metallization of the overlayer is brought about. Since the antibonding state can be viewed as a K 4s state, the observation of the antibonding state is direct evidence for the metallization of the K overlayer.