Analytical methods

Schematic presentation of the processes involved in XPS, UPS, and IPES.

Picture of the ultra-high vacuum surface-analysis system of the Young Investigator Group (April 2009).

Schematic presentation of the processes involved in XES and XAS.

Synchrotron-Based Characterization

In addition to our lab-based techniques (PES, IPES), we also use synchrotron-based characterization methods. The synchrotron-based characterization efforts are based on regular experimental campaigns at the synchrotron light source in Berlin (BESSY II) and in Berkeley (Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA).

X-ray Emission Spectroscopy

In addition to our lab-based techniques (PES, IPES), we also use synchrotron-based characterization methods.

As PES, the x-ray emission spectroscopy (XES) probes occupied states and hence can be used to investigate the chemical and electronic structure of samples. For XES a core-level electron is excited by (soft) x-ray photons. The created core hole is filled by an outer-level (most often a valence) electron, causing the emission of a photon, which is detected by an x-ray spectrometer. The cross section for this process (compared to that of, e.g., direct photoemission) is very small, demanding the experiments to be performed at high-flux synchrotron light sources. Because the radiative relaxation process takes place with respect to the created core hole, XES is an excellent tool to probe the local structure of a specific atomic center.

Since XES (contrary to PES) is a photon-in/photon-out technique, the information depth is roughly two orders of magnitude larger than that of PES.

When the core-level electron is excited to the lowest unoccupied state of the samples conduction band, it is referred to as resonant inelastic x-ray scattering (RIXS). RIXS is used to map the electronic sample structure and is therefore uniquely suited to “test” calculated band structures.

X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) investigates unoccupied states. It is an element-specific technique, sensitive to bonding environment and geometry.  XAS involves the excitation of core-level electrons into unoccupied states of the sample's conduction band. For these measurements, the excitation energy is varied in distcrete intervals around the core absorption level of a specific element in the sample. The fraction of absorbed photons at each energy step is recorded through monitoring of secondary processes, eg. radiative maintain charge neutrality (electron yield, EY). Depending on what detection channel is used, the XAS information depth is similar to that of XES (when using FY) or of PES (when using EY).

Similar to PES and IPES, also XES and XAS can be combined to reveal the band gap* of the near-surface bulk of the sample.