The Structure of Molecular Orbitals Investigated by Angle-Resolved Photoemission

Abstract

In this contribution, it is shown how the combination of angle-resolved photoemission spectroscopy (ARPES) with ab-initio electronic-structure calculations within the framework of density-functional theory (DFT) leads to insights into electronic and structural properties of organic molecular layers well beyond conventional density-of-sates or E(k) investigations. In particular, we emphasize the rather simple, but for many cases sufficiently accurate, connection between the observed angular dependence of the photocurrent with the spatial distribution of the molecular orbital from which it is arising. After discussing the accuracy and limitations of this approach, which is based on a plane-wave approximation of the final state, three examples are presented. The first utilizes the characteristic angular pattern of the highest occupied molecular orbitals (HOMO) in a pentacene multilayer film in order to measure the molecular tilt angle in the film. In the second example, the nature of two closely spaced molecular emissions from a porphyrin thin film is unambiguously identified as HOMO and HOMO-1, and the molecule’s azimuthal alignment is determined. Finally, for a monolayer of para-sexiphenyl on Cu(110), it is demonstrated how the real-space distribution of the filled LUMO and the HOMO of para-sexiphenyl can be reconstructed from the angular dependence of the photocurrent.