Progress in HAXPES performance combining full-field k -imaging with time-of-flight recording

Abstract

An alternative approach to hard-X-ray photoelectron spectroscopy (HAXPES) has been established. The instrumental key feature is an increase of the dimensionality of the recording scheme from 2D to 3D. A high-energy momentum microscope detects electrons with initial kinetic energies up to 8keV with a k-resolution of 0.025Å -1, equivalent to an angular resolution of 0.034°. A special objective lens with k-space acceptance up to 25Å -1 allows for simultaneous full-field imaging of many Brillouin zones. Combined with time-of-flight (ToF) parallel energy recording this yields maximum parallelization. Thanks to the high brilliance (10 13 hνs -1 in a spot of <20μm diameter) of beamline P22 at PETRAIII (Hamburg, Germany), the microscope set a benchmark in HAXPES recording speed, i.e. several million counts per second for core-level signals and one million for d-bands of transition metals. The concept of tomographic k-space mapping established using soft X-rays works equally well in the hard X-ray range. Sharp valence band k-patterns of Re, collected at an excitation energy of 6keV, correspond to direct transitions to the 28th repeated Brillouin zone. Measured total energy resolutions (photon bandwidth plus ToF-resolution) are 62meV and 180meV FWHM at 5.977keV for monochromator crystals Si(333) and Si(311) and 450meV at 4.0keV for Si(111). Hard X-ray photoelectron diffraction (hXPD) patterns with rich fine structure are recorded within minutes. The short photoelectron wavelength (10% of the interatomic distance) 'amplifies' phase differences, making full-field hXPD a sensitive structural tool.