Photoelectron Spectroscopy

Photoelectron spectroscopy (photoemission) is one of the cornerstones of modern material science and one of the central experimental techniques used by users of BESSY II and by the researchers in this institute. The technique is based on the photo effect: A photon transfers its energy to the sample and thereby lifts an electron into an excited state, from which it can escape from the sample. From the energy distribution of the photoelectrons, which is probed in an electron energy spectrometer the fundamental energy levels and excitation energies in the sample are determined. Different energy ranges give access to different information: Electrons emitted from core levels provide information about the chemical composition and oxidation state; photoemission with focus on these questions is called ESCA (electron spectroscopy for chemical analysis).

Electrons from weakly bound states allow one to probe the band structure in solids. In angle-resolved photoemission (ARPES) the information about the electron momentum in the sample is partially recovered and with spin-sensitive detectors all quantum numbers of the electron can be determined. Interactions between the emitted electron and its environment are directly reflected in a photoemission experiment, since part of the energy of the photon is transferred to this environment. The momentum distribution of the most weakly bound electrons in a solid defines the Fermi surface. Its determination is an important experimental check for theoretical models in strongly correlated materials like high-temperature superconductors or topological insulators.

Photoelectron spectroscopy is a traditionally a surface sensitive technique, which requires preparation of the samples in ultrahigh-vacuum and frequent sample exchange. New experimental developments have lifted this constraint. Photoemission at high kinetic energies excited with hard x-ray photons (HAXPES or HIKE) is a truly bulk sensitive spectroscopy. The BESSY-II beamline KMC1 offers the additional opportunity to tune the probing depth over a wide range. A planned upgrade project 60to6 will drive this tunability to the ultimate limit such that a probing depth between 4 Å and 100 Å can be chosen without even changing the experimental geometry. Recent advances in spectrometer design allow for a hundred times higher transmission and novel spin-filters surpass conventional systems by orders of magnitude in efficiency. Such new instruments reduce the acquisition time drastically. The furthermore allow for the first time to study radiation sensitive samples like organic crystals. The Institute for Methods and Instrumentation for Synchrotron Radiation Research is pushing these developments forward. In a collaboration with Uppsala University and the Swedish Company vg-Scienta the ARTOF instrument has been developed, which is an ultrahigh-transmission angle-resolving photoelectron spectrometer based on a time-of-flight detection scheme. A new project with the group of Prof. Schönhense at Mainz University, Prof. Kirschner at the Max-Planck-Institute in Halle and Prof. Weinelt at the Freie Universität Berlin aims at the installation at an ultrahigh-transmission experiment with spin filter. Also the 60to6 project will be equipped with an ultra-efficient spin filter. Another important progress is the ability to do photoemission from liquids. This experimentally challenging method has been developed at HZB in the past years.