METAL-DEPOSITION ON OXIDE SURFACES - A QUANTUM-CHEMICAL STUDY OF THE INTERACTION OF RB, PD, AND AG ATOMS WITH THE SURFACE VACANCIES OF MGO

The interaction of Rb, Pd, and Ag atoms with the surface vacancies of MgO, the F-s and the V-s centers, has been studied by ab initio cluste r model wave functions. We have considered the. interaction of each at om with F-s, F-s(+), F-s(2+), V-s, V-s(-), and V-s(2-) sites. These si tes correspond to the removal of O, O-, O2-, Mg, Mg+, and Mg2+ atoms o r ions, respectively, from the surface. The bond with the metal atoms, which is found to be very weak on the regular surface sites, can be v ery different depending of the formal charge of the vacancy. Neutral F -s centers are in general rather unreactive as their electronic struct ure resembles that of the regular surface; F-s(+) paramagnetic centers have a relatively large electron affinity and tend to ionize metal at oms with low ionization potentials, such as alkali-metal atoms or to f orm covalent polar bonds with the adsorbed metal atoms; F-s(2+) center s have a very high electron affinity so that all metal atoms are ioniz ed when interacting with these sites. Neutral V-s sites are also elect ron deficient; here the metal atoms tend to form a dication and to rep lace the missing Mg ion in the lattice with large gain in the electros tatic energy. On V-s(-) vacancies the metal atoms lose one electron an d become singly ionized with formation of strong ionic bonds al the in terface. Finally. no charge transfer occurs between the metal atoms an d the electronically saturated V-s(2-) sites; in this case the bond st rength is due only to the metal polarizability.