Investigating the interaction between π-conjugated organic molecules and metal surfaces with photoemission tomography

Photoemission tomography (PT) is a combined experimental and theoretical technique applied to molecule–metal interfaces which uses angle-resolved photoemission spectroscopy over a wide angular range, while the photoelectron angular distributions in reciprocal space (momentum maps, or called k-maps) are interpreted in terms of the molecular orbital structure of the initial state. This thesis uses PT to investigate various aspects of the interaction between π-conjugated organic molecular adsorbates and metal surfaces: PT was successfully used to identify the exact products of chemical reactions at surfaces and their local bonding. The measured k-maps confirm a modification of the orbital structure of dibromo-bianthracene on Cu(110) in the thermal reaction and the fully hydrogenated bisanthene is found to be the correct reaction intermediate. To decouple molecular adsorbates from the metal substrate, PT was employed to gauge whether charge is transferred through the interface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu–O covalent bonds, thus achieving electronic and physical decoupling of perylene-tetracarboxylic-dianhydride as determined by combined results of PT and normal incidence X-ray standing waves. A special example of an electronically inhomogeneous unary molecular layer on a metal surface is showcased in the saturated monolayer of tetracene on Ag(110). With the help of PT, two highest occupied molecular orbital peaks in the photoemission spectra were found, indicating that two molecular species coexist in the tetracene layer. While one molecule remains neutral, another is charged. Finally, we applied PT to study photoelectron angular distributions for highly-hybridized molecule–metal systems, monolayers of p-sexiphenyl, p-quinquephenyl, and pentacene on Cu(110) and on Ag(110), respectively. In k-maps measured for the lowest unoccupied molecular orbital , PT has identified the scattering of either the Shockley surface states or the states around the projected bulk band gap. The scattering vectors can be directly related to reciprocal lattice vectors of the overlayer structure