Valence band photoemission from the Zn-phthalocyanine/Ag(110) interface: Charge transfer and scattering of substrate photoelectrons

Abstract

The electronic properties of Zn-phthalocyanine vacuum deposited on Ag(110) are studied by high-resolution valence-band photoelectron spectroscopy. Submonolayer, one ordered monolayer, and molecular layers of increasing thickness present spectral features that can be related to the molecule-substrate interaction as well as to the effect of the overlayer on the escape conditions of the substrate photoelectrons. For the first-ordered molecular layer, an interface state related to a charge transfer from the substrate to the molecules is detected through the appearance of a new feature at low binding energy. Such feature, appearing already at submonolayer coverage, is interpreted as the partial filling of the lowest unoccupied molecular orbital (LUMO) and to possible hybridization of the LUMO with substrate states. Because it is the major change observed in the molecular electronic structure after adsorption, the filling of the LUMO is likely to have a major role in the molecular chemisorption. The spectral line shape of the filled part of the LUMO is discussed in terms of electron-vibron coupling and electron correlation effects. Molecular states lying at higher binding energy show spectral modifications as a function of coverage implying a weaker contribution in the molecule-substrate interaction. Important changes are found in the spectral region of the $\text{Ag}\text{ }4d$ band after the deposition of a submonolayer and as a function of coverage although no interface states are detected. It is shown that these features can be entirely explained by considering the effect of the molecular overlayer on the escape conditions of the substrate photoelectrons. The major effect is the emerging of the substrate three-dimensional density of states. Such evidence is not simply related to the present case but it is expected to apply to other systems where an organic layer is deposited on a single-crystal substrate.