Using photoelectron diffraction to determine complex molecular adsorption structures

Abstract

Backscattering photoelectron diffraction, particularly in the energy-scan mode, is now an established technique for determining in a quantitative fashion the local structure of adsorbates on surfaces, and has been used successfully for ~100 adsorbate phases. The elemental and chemical-state specificity afforded by the characteristic core level photoelectron binding energies means that it has particular advantages for molecular adsorbates, as the local geometry of inequivalent atoms in the molecule can be determined in a largely independent fashion. On the other hand, polyatomic molecules present a general problem for all methods of surface structure determination in that a mismatch of intramolecular distances with interatomic distances on the substrate surface means that the atoms in the adsorbed molecule are generally in low-symmetry sites. The quantities measured experimentally then represent an incoherent sum of the properties of each structural domain that is inequivalent with respect to the substrate point group symmetry. This typically leads to greater ambiguity or precision in the structural solutions. The basic principles of the method are described and illustrated with a simple example involving molecule/substrate bonding through only one constituent atom (TiO2-(110)/H2O). This example demonstrates the importance of obtaining quantitative local structural information. Further examples illustrate both the successes and the problems of this approach when applied to somewhat more complex molecular adsorbates.