  Measuring the delay for an electron to emerge from different states is one of
the major achievements of attosecond science. This delay can have two origins -
the electron wave packet is reshaped during departure by the electrostatic
field of the ionizing medium or it is modified by dynamic interaction with the
remaining electrons. Most experiments have observed the former, but
confirmation requires a complex calculation. A direct measurement of
multielectron dynamics is needed. Photo-recombination - the inverse of
photoionization - occurs naturally during electron recollision and can be
measured by combining a perturbing beam to modify the recollision electron
before recombination. These in situ methods allow us to unambiguously isolate
multielectron dynamics - the reference being the spectral phase of an
attosecond pulse simultaneously measured in spectral regions without
multielectron interaction. Here, we measure the group delay of the recollision
electron caused by plasmonic resonance dynamics in Xe, simulate the in situ
measured spectral phase of a recollision electron generated in the presence of
the plasmonic resonance in C$_{60}$ and present a corresponding semi-classical
theory based on the strong-field approximation. Our results suggest that in
situ techniques, together with 300 eV recollision electrons, will allow the
ultimate time response of electronic matter to be measured.
