Photoelectron spectra as a function of BE (with respect to the first ionization potential, D 0 ) and pump-probe time delay between UV and XUV pulses, plotted as the electron yield difference between spectra taken with and without the UV-excitation pulse (Extended Data Fig. 1 ). The respective electron yield at each delay is normalized to the FEL pulse energy before subtraction, and the difference signal is normalized to the integrated electron yield without UV excitation, both summed over all shots at the respective delay. Negative delays correspond to the FEL pulse preceding the UV excitation pulse.
