Time-of-flight photoelectron spectrometer for condensed-phase photoemission
The schematic above depicts the operational principle of the SPECS Themis 600 time-of-flight spectrometer. A set of electrostatic lenses (L1 – L10) creates an electric field that guides photoelectrons emitted from the sample to the combined position-sensitive micro-channel plate (MCP) and delay-line detector (DLD). The photoelectron signal is multiplied by the position-sensitive MCP and the subsequent DLD is triggered synchronously with the arrival of laser pulses at the sample interaction region, providing a timing reference to measure the electron flight time. The AUX grid in front of the detector can be biased at a negative potential to serve as a high-pass energy filter and reject low kinteic energy electrons, preventing saturation of the detector. Correspondingly, using the associated DLD amplification and timing electronics, it is possible to record a 3D momentum distribution of all photoelectrons that enter the spectrometer with kinetic energies greater than a user-set threshold value.
The electrostatic fields imposed on the generated photoelectrons permits an efficient signal collection. An acceptance angle of electron detection up to +/-15° can be achieved, though the highest energy resolution is reached with the field-free configuration and lower acceptance ranges (+/-~1°). Different predefined modes of the electrostatic field – including the field-free configuration – can be applied, allowing the acceptance angle, angular resolution, and the energy resolution to be tailored to different experiments. Taking into account the chosen electrostatic field configuration, software is used to reconstruct the electron trajectory from the measured flight time and its arrival coordinates at the 2D detection plane. As a result, the original kinetic energy and the emission angle of the detected photoelectrons can be derived.
The entrance to the spectrometer system houses a skimmer with a small orifice. This allows the electrons emitted from volatile samples to be collected and analyzed while maintaining the high-vacuum conditions required for electron detection beyond the entrance skimmer. In particular, the spectrometer is used in experiments on liquid solutions, introduced to the interaction region in the form of a microjet.