Asymmetric electrostatic dodecapole: compact bandpass filter with low aberrations for momentum microscopy

Imaging energy filters in photoelectron microscopes and momentum microscopes employ spherical fields with deflection angles of 90{\deg}, 180{\deg} and even 2 x 180{\deg}. These instruments are optimized for high energy resolution, yet they come along with image aberrations when they are operated in high transmission mode with medium energy resolution. Here we present a new approach for bandpass-filtered imaging in real or reciprocal space, using an asymmetric electrostatic dodecapole. This multipole enables energy-dispersive beam deflection and correction of image aberrations up to the 3rd order. Owing to a deflection angle of only 4{\deg}, the total beam displacement in the filter is just ~10 mm. Hence, the entire instrument is compact and just requires a straight vacuum tube. The multipole is framed by transfer lenses in the entrance and exit branch. Two sets of 16 entrance and exit apertures with different sizes on piezomotor-driven holders allow selecting the desired resolution. The combination of apertures and dodecapole acts as a bandpass pre-selector in a high-energy time-of-flight momentum microscope at the hard X-ray beamline P22 at PETRA-III (DESY, Hamburg). At pass energies between 400 and 600 eV it transmits electrons with kinetic energies in the range of 20-40 eV and thus effectively eliminates unwanted intensity from higher-energy electrons in the ToF analyzer. At low pass energies, the instrument allows energy-filtered imaging without subsequent ToF analysis. In a laboratory experiment the 4{\deg} prototype reached<500 meV resolution, which is sufficient for fast survey studies in the X-ray range.