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Sketch of the experimental setup. The two ToF spectrometers are aligned coplanar ±45° with respect to the sample normal. Here, for (γ,2e), the HHG light source is driven by a 1040 nm laser that is focused in a gas jet. The generated high-order harmonics passes through a monochromator for photon energy selection. Inset: Parallel momentum distribution of photoelectrons from the Shockley surface state as calibration of the ToF spectrometers.
2D energy distribution obtained with Ep = 27 eV on Cu(111). The dashed white line marks the onset of pair emission. Three features emerge at 0.5, 2.4, and 4.8 eV below the onset. The inset shows the parallel momentum distribution for pairs between and . The corresponding state is marked with the dashed ellipse in the energy distribution.
Sum energy vs. sum momentum in kx with . Directly below the emission onset, a feature with parabolic dispersion appears. The offset to zero in kx is due to a small sample misalignment.
(a) 2D energy distribution obtained with on NiO(001). The diagonal dashed line marks the expected onset of pair emission. The intensity does not reproduce that but increases slowly towards lower pair energies. Also no preference for a specific energy sharing can be distinguished. (b) Intensity depending on Esum along the white rectangle in (a).
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