  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.
