INTERACTION OF SILVER, CESIUM, AND ZINC WITH ALUMINA SURFACES - THERMAL-DESORPTION AND PHOTOEMISSION-STUDIES

Cs, Ag, and Zn behave in a different way when deposited on surfaces of ultrathin (15-20 Angstrom) films of alumina. In the case of Cs/Al2O3, the alkali atoms wet the oxide, forming strong chemisorption bonds. A t small coverages of Cs, desorption of the alkali occurred between 600 and 1000 K. As the amount of Cs deposited increased, there was a redu ction in the Cs desorption temperature to similar to 350 K. This broad range of desorption temperatures reflects a continuous decrease in th e Cs adsorption energy from 63 to 18 kcal/mol with increasing Cs cover age. The deposition of small amounts of Cs induced large (0.9-1.1 eV) positive shifts in the binding energies of the O KVV, O 1s, and Al 2p features of the alumina films. This is consistent with a transfer of e lectrons from Cs into surface states of alumina that produces a transf ormation similar to a change from p-type to n-type semiconductors, AE and Zn do not wet the surface of alumina well and form three-dimension al clusters or particles. In these systems, the activation energies fo r desorption of the admetal increase with cluster size (20 kcal/mol fo r Ag, 10 kcal/mol for Zn). The deposition of Ag and Zn induces only mi nor perturbations in the electronic properties of alumina. Small silve r clusters supported on alumina exhibit an incompletely developed meta llic band structure. The differences in the behavior of the Cs/Al2O3 a nd Ag/Al2O3 or Zn/Al2O3 systems are explained in terms of variations i n the surface free energy of the admetals and in the nature of the adm etal-oxide bonds.