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

• Published in: Journal of synchrotron radiation Vol. 26; no. 6; pp. 1996 - 2012
• Main Authors: Medjanik, K., Babenkov, S. V., Chernov, S., Vasilyev, D., Schönhense, B., Schlueter, C., Gloskovskii, A., Matveyev, Yu, Drube, W., Elmers, H. J., Schönhense, G.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01.11.2019; John Wiley & Sons, Inc
• Subjects: Angular resolution; Brillouin zone; Brillouin zones; Diffraction patterns; Energy measurement; Fine structure; HAXPES; Interatomic distance; k‐space; Mapping; Photoelectrons; Recording; Research Papers; time‐of‐flight microscope; Transition metals; Valence band; X‐ray photoelectron diffraction
• ISSN: 1600-5775; 0909-0495; 1600-5775
• DOI: 10.1107/S1600577519012773

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 8 keV with a k‐resolution of 0.025 Å−1, equivalent to an angular resolution of 0.034°. A special objective lens with k‐space acceptance up to 25 Å−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 (1013 hν s−1 in a spot of <20 µm diameter) of beamline P22 at PETRA III (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 6 keV, correspond to direct transitions to the 28th repeated Brillouin zone. Measured total energy resolutions (photon bandwidth plus ToF‐resolution) are 62 meV and 180 meV FWHM at 5.977 keV for monochromator crystals Si(333) and Si(311) and 450 meV at 4.0 keV 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. A highly effective way to cope with the weak signals in hard X‐ray angular‐resolved photoelectron spectroscopy is introduced. Full‐field momentum imaging combined with time‐of‐flight parallel energy recording constitute a 3D recording scheme, gaining two orders of magnitude in detection efficiency.