THEMIS 1000 2D-DLD

We discuss some experimental facets of electron pair emission from surfaces using two different experimental
approaches. In the first case the instrument consists of a pair of hemispherical analyzers which are operated
with continuous primary beams of electrons or photons. The second instrument employs a pair of time-of-flight
spectrometers which require a pulsed excitation source. A key experimental quantity is the ratio of ‘true’ to
‘random’ coincidences which can be determined in different ways. Regardless of the type of instrument the
primary flux has to adopt a much smaller value than in single electron spectroscopy. We describe different
approaches to obtain the relevant count rates, in particular the concept of operating with a delayed coincidence
circuit. We also address the question on how to compare the two types of spectrometer in terms of their
performance.

R. Kamrla, W. Widdra, C.-T. Chiang, and F.O. Schumann
Journal of Electron Spectroscopy and Related Phenomena 257 (2022) 147185

The recent development of double photoemission (DPE) spectroscopy at surfaces using laserbased
high-order harmonic generation in combination with time-of-flight electron spectroscopy
is reviewed. Relevant experimental conditions including the solid angle for collecting photoelectron
pairs, the energy and angular resolutions, as well as the repetition rate and the photon
energy range of light sources are introduced. As examples, we provide an overview of laser-based
DPE results on the noble metals Ag and Cu as well as transition metal oxides NiO and CoO. The
DPE energy and angular distributions of photoelectron pairs are compared with emphasis on the
possible indications of electron-electron interaction. Potential further developments including
femtosecond time-resolved DPE experiments are outlined.

C.-T. Chiang, A. Trützschler, M. Huth, R. Kamrla, F. O. Schumann, und W. Widdra,
Progress in Surface Science 95 (2020) 100572

We present results for electron coincidence spectroscopy using two time-of-flight (ToF) spectrometers. Excited by electron impact, the energy and momentum distribution of electron pairs emitted from the Cu(111) surface are resolved and a spectral feature related to the Shockley surface state is identified. By combining the two ToF spectrometers with a high-order harmonic generation light source, we demonstrate double photoemission spectroscopy in the laboratory that required synchrotron radiation in the past. Utilizing this setup, we report results for (γ,2e) on NiO(001) on Ag(001) excited with light at 30 eV photon energy.

M. Huth, C.-T. Chiang, A. Trützschler, F.O. Schumann, J. Kirschner,W. Widdra
Appl. Phys. Lett. 104, 061602