Resonance effects in photoemission time delays

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We present measurements of single-photon ionization time delays between valence electrons of argon and neon using a coincidence detection technique that allows for the simultaneous measurement of both species under identical conditions. Taking into account the chirp of the ionizing single attosecond pulse (attochirp) ensures that the clock of our measurement technique is started at the same time for both types of electrons, revealing with high accuracy and resolution energy-dependent time delays of a few tens of attoseconds. By comparing our results with theoretical predictions, we confirm that the so-called Wigner delay correctly describes single-photon ionization delays as long as atomic resonances can be neglected. Our data, however, also reveal that such resonances can greatly affect the measured delays beyond the simple Wigner picture.