  Amplitude and phase of wavepackets encode the dynamics of quantum systems.
However, the rapidity of electron dynamics on the attosecond timescale has
precluded their complete measurement in the time domain. Here, we demonstrate
that spectrally-resolved electron interferometry reveals the amplitude and
phase of a photoelectron wavepacket created through a Fano autoionizing
resonance in helium. Replicas obtained by two-photon transitions interfere with
reference wavepackets formed through smooth continua, allowing the full
temporal reconstruction, purely from experimental data, of the resonant
wavepacket released in the continuum. This in turn resolves the buildup of the
autoionizing resonance on attosecond timescale. Our results, in excellent
agreement with ab initio time-dependent calculations, raise prospects for both
detailed investigations of ultrafast photoemission dynamics governed by
electron correlation, as well as coherent control over structured electron
wave-packets.
