  When intense light irradiates a quantum system, an ionizing electron
recollides with its parent ion within the same light cycle and, during that
very brief (few femtosecond) encounter, its kinetic energy sweeps from low to
high energy and back. Therefore, recollision offers unprecedented time
resolution and it is the foundation on which attosecond science is built. For
simple systems, recolliding trajectories are shaped by the strong field acting
together with the Coulomb potential and they can be readily calculated and
measured. However, for more complex systems, multielectron effects are also
important because they dynamically alter the recolliding wave packet
trajectories. Here, we theoretically study Fano resonances, one of the most
accessible multielectron effects, and we show how multielectron dynamics can be
unambiguously isolated when we use in situ measurement. The general class of in
situ measurement can provide key information needed for time-dependent ab
initio electronic structure theory and will allow us to measure the ultimate
time response of matter.
