Photoelectron diffraction in the x-ray and ultraviolet regime: Sn-phthalocyanine on Ag(111)

Abstract

The bonding geometry of tin-phthalocyanine (SnPc) on Ag(111) has been studied using x-ray and ultraviolet photoelectron diffraction (XPD and UPD, respectively). Experimental diffraction patterns were compared to single-scattering-cluster calculations. XPD data could be well reproduced by the simulations and allowed for the determination of several structural parameters. At a coverage of 0.9 ML, all molecules are in a ``tin-down'' configuration and the nonplanar shuttlecock-shaped SnPc molecule undergoes flattening upon absorption on Ag(111). UPD data from the second highest occupied molecular orbital and comparison to simulations show a high sensitivity to minor structural changes, including also the vertical distance between tin atoms of the SnPc and the surface layer of the substrate, which is found to be 2.3 \AA{}. We thus demonstrate how UPD can complement the well-established XPD method and discuss remaining challenges in the theoretical description of photoelectron diffraction from molecular orbitals at low energies. The UPD method is particularly attractive in view of the increasing availability of ultrashort pulsed laser sources in the XUV regime, which could enable pump-probe experiments with high structural sensitivity.