Structural investigation of the adsorption of SnPc on Ag(111) using normal-incidence x-ray standing waves

The bonding of tin(II)-phthalocyanine (SnPc) on Ag(111) was studied using the normal incidence x-ray standing wave technique. For an incommensurate monolayer structure at room temperature, it was found that the SnPc molecules adsorb in a ``Sn-down'' geometry, i.e., the Sn atoms lie below the molecular plane. A significant bending of the benzene rings toward the surface indicates that these rings contribute to a chemisorptive bonding of the molecule to the surface. This effect is found to be even enhanced for another phase, a commensurate submonolayer structure that is only stable at low temperature. In this phase, the molecules are located significantly closer to the surface in a mixed ``Sn-up''-``Sn-down'' configuration. Nondipolar contributions to the photoelectron yield were also investigated and taken into account.