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Energy dispersion of electron bunches photoemitted from (a) a guard ring cathode and (b) a truncated tip cathode at increasing UV fluence (blue 7.3 μJ/cm2, red 62 μJ/cm2, yellow 740 μJ/cm2). The corresponding average numbers of electrons per pulse measured at the retractable CCD camera are shown in panels (c) for the guard ring cathode and (d) for the truncated tip cathode.
