Initial and Final State Effects in the Ultraviolet and X-ray Photoelectron Spectroscopy (UPS and XPS) of Size-Selected Pdn Clusters Supported on TiO2(110)

Photoelectron spectroscopy is a powerful tool for investigating the electronic structure of supported clusters, especially when initial state and final state contributions to the electron binding energy can be separated. We have performed a combined experimental and theoretical: study of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) using atomically se-selected Pd-n dusters on rutile TiO2(110). Theoretical investigations allow for the UPS and XPS shifts to be split into initial state and final state contributions. XPS, the occupation of the 4d orbital of Pd controls the initial state shift offering information about the hybridization of the cluster, while the size and the charging of the duster controls the final state shift. In UPS, we evaluate two methods for calculating the final state shift in periodic unit cells and find that both methods give reasonable results for pristine TiO2; however, using a p-type dopant fails when two separate donor acceptor pairs are present. The observed UPS shifts can be described by combining the surface dipole and the final state shifts. Metallic contacts to the semiconductor surface result in band alignment between the metallic contact and the cluster; shifting the Fermi level to lie just below the conduction band of the TiO2. Information about the charge state and hybridization of the cluster are revealed by separating the initial and final state effects.