PHOTON SCIENCE

WESPE = Wide-angle Electron SPEctrometer

Figure 1: CAD image of WESPE

The WESPE end-station is a versatile and transportable experiment, operable on either one of the two monochromator beamlines, PG2 and FL23, at the Free-Electron Laser in Hamburg (FLASH) or in the laser laboratory at the Center for Free-Electron Laser Science (CFEL).

The experiment is equipped with a prep chamber providing standard techniques for sample preparation. The measurement chamber is constructed from µ-metal to effectively shield external magnetic fields. In its current configuration, the setup offers two operational modes:

1. High-repetition rate measurements at FLASH

The setup combines time-of-flight analyzers (Specs, Themis 1000-80) with segmented, position-sensitive delay-line detectors (Surface Concept, 3D-DLD-4Q), providing optimal conditions to perform high-repetition-rate measurements. By using FLASH, as a soft X-ray free-electron laser with excitation energies up to 700 eV (wavelengths down to 1.77 nm), ideal conditions are provided to investigate a variety of time-resolved (core-level) photoemission experiments (trPES) on solid surfaces and interfaces, such as:

• Catalytic reactions on solid surfaces or Nanoparticles,
• Ultrafast magnetization phenomena,
• Charge migration and transfer processes in layered systems.

The segmented delay-line detector (DLD) enhances detection efficiency, supporting single-electron detection. Additionally, the wide acceptance angle (±14°) and high energy resolution (~150 meV) across a large energy window (up to 18 eV) from the electron analyzer contribute to superior measurement capabilities. The parallel operation of two spectrometers enables:

1. Simultaneous bulk- and surface-sensitive measurements, with one spectrometer in normal emission (NE) and the other in grazing emission (GE).
2. Measurement of electrons with different kinetic energies.
3. Improved statistical reliability through parallel data acquisition.

2. Static and Preparatory Measurements

A hemispherical analyzer (Scienta SES 2002) is also integrated into the setup. Operable in conjunction with a twin-anode X-ray gun (Al- and Mg- K_α), this configuration supports:

• Stand-alone static experiments in the laboratory,
• preparatory measurements for FEL experiments.

Figure 2: WESPE at P2