Electron Spectroscopy Group

Photoionization and Autoionization of Clusters

Clusters are of interest as such, since they represent a link between isolated, gaseous atoms or molecules, and the infinite bulk. For hydrogen bonded systems, such as water, they serve as important models for the liquid. The electronic structure of clusters, as of other systems, can be accessed by photoelectron spectroscopy.

In the "Electron Spectroscopy" group, free clusters are created from a supersonic expansion of sample gas into vacuum. The clusters are ionized by the synchrotron radiation of BESSY. Different analysers to detect the created photoelectrons and electrons from secondary processes are used.

Photoelectron spectrum of the highest occupied orbital in water clusters. When the clusters grow in size, the ionization potential of the orbital shifts towards lower values. This can be explained by the increasing polarizability of larger clusters.
Valence ionization of water clusters: From isolated molecules to bulk

Photoelectron-Auger electron coincidence studies

Auger spectroscopy is widely used as an element specific probe in material science. More information, e.g. about potential curves of the final state and about dynamics in the intermediate state is contained in the Auger spectrum in principle. This information is difficult to extract though, because the number of dicationic final states typically is large. Moreover an Auger spectrum, conventionally recorded, is composed of a convolution of all accessible intermediate states. A technique that can overcome this natural barrier is two electron coincidence spectroscopy.

The "Electron Spectroscopy" group has developed an apparatus for electron-electron coincidence experiments that acheives a high energy resolution. A recent example for a spectrum of inner shell photoelectron-Auger electron pairs from a small molecule is shown in the next figure.

Electron-Electron coincidence spectrum of CO at 304 eV kinetic energy. The vibrational structure in the dicationic a state (the least energetic singlet state of CO2+) can be clearly resolved around an Auger energy of 255 eV. For the d state, transitions between pairs of vibrationally resolved intermediate and final states are clearly separated as well. As they appear at practically identical Auger energy, it can be concluded that the potential curves of both states are parallel. The apparatus broadening for these data corresponds to 200 x 200 meV for the photoelectron and the Auger electron, resp.
Separating the Vibrationally Resolved Auger Decay Channels for a CO Core Hole State