  Streaked photoemission metrology allows the observation of an apparent
relative time delay between the detection of photoelectrons from different
initial electronic states. This relative delay is obtained by recording the
photoelectron yield as a function of the delay between an ionizing ultrashort
extended ultraviolet (XUV) pulse and a streaking infrared (IR) pulse.
Theoretically, photoemission delays can be defined based on i) the phase shift
the photoelectron wavefunction accumulates during the release and propagation
of the photoelectron (``Wigner delay") and, alternatively, ii) the streaking
trace in the calculated photoemission spectrum (``streaking delay"). We
investigate the relation between Wigner and streaking delays in the
photoemission from atomic and solid-surface targets. For solid targets and
assuming a vanishing IR-skin depth, both Wigner and streaking delays can be
interpreted as an average propagation time needed by photoelectrons to reach
the surface, while the two delays differ for non-vanishing skin depths. For
atomic targets, the difference between Wigner and streaking delays depends on
the range of the ionic potential.
