Interpretation of a new feature in photoemission from Cu based alloys with polyvalent solutes

In our recent angle resolved photoemission studies of oriented single crystal surface of $\text{Cu}$Al and $\text{Cu}$Ge solid solutions we had found a new structure lying between 4 and 5 eV below the Fermi energy, which could not be related to either the bulk or the surface states in the random alloy. To understand its nature and origin we report and discuss photoemission measurements from Cu films, submonolayer to a monolayer thick, deposited on the Al(111) and $\text{Cu}\text{Al(111)(}\text{3}\text{×}\text{3}\text{R}\text{30°)}$ alloy surfaces and from Al and Zn alloys containing dilute Cu impurities. Some experiments on the (100) surfaces in the various cases were also carried out. On the basis of these results, we suggest that the aforementioned new photoemission feature is characteristic of Cu clusters and isolated Cu impurities in polyvalent impurity-rich environment. These clusters probably lie at the topmost layer of the $\text{Cu}$Al and $\text{Cu}$Ge alloy surfaces.