EQUILIBRIUM AND NONEQUILIBRIUM SURFACE-STRUCTURES OF AL PD(001)/

It has been demonstrated that the equilibrium state of Al deposited on Pd (0 0 1) and annealed to 900 K is a (2 x 2)p4g structure induced by the stress related to the formation of a pure Pd surface layer over a c(2 x 2) AI-Pd second layer. This state can be changed by exposure to oxygen (at room temperature) which causes the Al to migrate to the su rface layer with a concentration of up to 50% as measured by non-therm al removal of oxygen. In this process the surface is exposed to H-2 at 375 K and the oxygen is removed. The oxygen-covered surface assumes a (1 x 1) structure. Once this surface is annealed to 700 K the oxygen is desorbed. Annealing to higher temperatures (900 K) returns the surf ace to the initial (2 x 2)p4g structure. This cycle has been followed for five cycles without significant loss of Al demonstrating the trapp ing power of the second layer structure. (C) 1998 Published by Elsevie r Science B.V.