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Single-Hemisphere Photoelectron Momentum Microscope With Time-of-Flight Recording

Photoelectron momentum microscopy is an emerging powerful method for angle-resolved photoelectron spectroscopy (ARPES), especially in combination with imaging spin filters. These instruments record kx-ky images, typically exceeding a full Brillouin zone. As energy filters double-hemispherical or time-of-flight (ToF) devices are in use. Here we present a new approach for momentum mapping of the full half-space, based on a single hemispherical analyzer (path radius 225 mm). Excitation by an unfocused He lamp yielded an energy resolution of 7.7 meV. The performance is demonstrated by k-imaging of quantum-well states in Au and Xe multilayers. The alpha-square-aberration term (alpha: entrance angle in the dispersive plane) and the transit-time spread of the electrons in the spherical field are studied in a large pass-energy (6 to 660 eV) and angular range (alpha up to about 7°). It is discussed how the method circumvents the preconditions of previous theoretical work on the resolution limitation due to the alpha-square-term and the transit-time spread, being detrimental for time-resolved experiments. Thanks to k-resolved detection, both effects can be corrected numerically. We introduce a dispersive-plus-ToF hybrid mode of operation, with an imaging ToF analyzer behind the exit slit of the hemisphere. This instrument captures 3D data arrays I(EB,kx,ky), yielding a gain up to N^2 in recording efficiency (N being the number of resolved time slices). A key application will be ARPES at sources with high pulse rates like Synchrotrons with 500 MHz time structure.