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Photoelectron statistics and quantum efficiency for tunable excitation. (A) A significant shrinking of the beam radius is observed toward lower excitation energies. The left scale provides the temporal broadening of electron pulses assuming an acceleration field of 10 kV/mm (see text). The red and blue lines denote conventional and optimized excitation wavelengths. (B) Quantum efficiency (electrons per photon, relative) in dependence of the photon energy (dots).
