A central goal of attosecond science is to visualize, understand and ultimately control electronic processes in matter over the fastest relevant timescales. To date, numerous schemes have demonstrated exquisite temporal resolution, on the order of ten attosecons, in measurements of the response of photo-excited electrons to time-delayed probes. However, attributing this response to specific dynamical mechanisms can be difficult, often requiring guidance from advanced calculations. Using a variant of method initially applied to characterize attosecond XUV pulses, we have shown that energy transfer between an oscillating field and low-energy attosecond photoelectron wavepackets directly provides coarse-grained information on the effective binding potential from which the electrons are liberated. That effective potential implicitly includes electron-electron as well as electron-ion interactions. Experimentally, in collaboration with the DiMauro group at the Ohio State University, we have employed a dense extreme ultraviolet (XUV) high-harmonic comb to photoionize He, Ne and Ar atoms and recorded the electron spectra as a function of the phase of a mid-infrared dressing field. The amplitude and phase of the resulting interference modulations in the electron spectra reveal the average momentum and change in momentum of the electron wavepackets during the first quarter-period (~ 1 fs) of the dressing field after their creation, providing a snapshot of the corresponding coarse characteristics of the effective binding potential from which the electrons were released. In principle, the method could be used as a time-resolved probe of changes in the effective binding potential induced through the action of other fields.
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