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

Imaging energy filters in photoelectron microscopes and momentum microscopes use spherical fields with deflection angles of 90°, 180° and even 2 × 180°. These instruments are optimized for high energy resolution, and exhibit image aberrations when operated in high transmission mode at medium energy resolution. Here, a new approach is presented for bandpass‐filtered imaging in real or reciprocal space using an electrostatic dodecapole with an asymmetric electrode array. In addition to energy‐dispersive beam deflection, this multipole allows aberration correction up to the third order. Here, its use is described as a bandpass prefilter in a time‐of‐flight momentum microscope at the hard X‐ray beamline P22 of PETRA III. The entire instrument is housed in a straight vacuum tube because the deflection angle is only 4° and the beam displacement in the filter is only ∼8 mm. The multipole is framed by transfer lenses in the entrance and exit branches. Two sets of 16 different‐sized entrance and exit apertures on piezomotor‐driven mounts allow selection of the desired bandpass. For pass energies between 100 and 1400 eV and slit widths between 0.5 and 4 mm, the transmitted kinetic energy intervals are between 10 eV and a few hundred electronvolts (full width at half‐maximum). The filter eliminates all higher or lower energy signals outside the selected bandpass, significantly improving the signal‐to‐background ratio in the time‐of‐flight analyzer. A compact bandpass prefilter eliminates electrons with energies above or below the desired range and can correct image aberrations up to the third order before the beam enters a time‐of‐flight analyzer. Here, the imaging performance of the filter is demonstrated for two key applications of high‐energy momentum microscopes: full‐field core‐level photoelectron diffraction and mapping of bulk valence bands.