Photoemission Tomography of Excited Molecular States

The aim of this project is to use photoemission tomography to study optically excited molecular states, so called excitons in organic semiconductors. Their formation, their transport through the molecular film and finally their decay are basic processes governing the performance of organic opto-electronic devices, such as photovoltaic cells. This project is a co-operation between the surface science group from the Institute of Physics, University of Graz and the quantum optics group from the University of Nova Gorica, Slovenia. The method photoemission tomography is an advancement of angle resolved ultraviolet- photoemission spectroscopy, which is based on the photoelectric effect. The photoelectric effect is the emission of electrons from a metallic surface upon irradiation with ultraviolet light. This effect has been discovered in the 19th Century and explained by Albert Einstein in 1905. In photoemission tomography one measures the photo-emitted electrons as a function of take-off angle for a specific kinetic energy and thus one can extract the structure of the emitting orbital. The molecular systems we want to study are the metal complexes of phthalocyanine and porphyrin molecules. These are ubiquitous in nature (chlorophylle, hemoglobin) and possess electronic and optical properties, which make them an attractive material group for photovoltaics or organic sensors. To measure now the orbital structure of excitons requires a so-called pump-probe set-up, i.e the molecules in the films will be excited (populated) by a first laser pulse. A second laser pulse (probe pulse) then provides the ultraviolet light pulse required for photoemission spectroscopy. To produce the ultraviolet probe laser light pulse requires high-order harmonic generation, where a high-power ultrafast Ti:Saphire laser is focused into a gas cell filled with a noble gas, generating higher harmonics of the fundamental laser frequency. Experimentally, we will use this set up to perform photoemission tomography of the excited states and determine their orbital structure. Utilizing the femtosecond time structure of the laser pulses and time resolved photoelectron detection we will monitor the temporal evolution of the exciton, in particular regarding changes in its orbital structure during its decay. These experiments will be accompanied by quantum-mechanical ab-initio calculations, which will support the interpretation of the results obtained.

Within the project we have investigated the frontier and excited state orbitals of acenes and porphyrins/phthalocyanins with the experimental method of photoemission orbital tomography (POT) and with theoretical modelling of time- and angle-resolved photoemission experiments, we have performed time-dependent density functional calculations using a real-space, real-time approach. Charge transfer states of porphyrins have been investigated on decoupling oxide surfaces and thin acene films. Within the course of the project the only recently available higher acene heptacene, which has a significantly lower band gap than the popular pentacene, has been investigated for the first time on coinage metal and oxide surfaces for the first time with POT. A particular interesting result was the connection between adsorption geometry, which could be varied via the substrate temperature during growth and the amount of charge transfer. From the theoretical view point, by applying time-dependent external electric fields accounting for the pump- and probe laser fields, the time- and angle-resolved photoemission current could be simulated in an ab-initio manner for a number of prototypical organic molecules. Importantly, we have also extended POT to excitons by retaining the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. This novel approach has been termed "exPOT".