The photoelectric effect explained by Einstein is often regarded as a one-electron phenomenon, whereas the interaction of the escaping electron with other electrons, referred to as electron correlation, plays an important role in multielectron systems. In this Letter, we study the attosecond photoionization of the outer $s$ subshell of argon in its substantial minimum cross-section region formed by electron correlation, which was theoretically predicted in 1972 and experimentally confirmed using synchrotron radiation. Combining high-spectral resolution attosecond interferometry experiments and novel theoretical calculations allows us to identify the most essential electron correlations affecting the time of photoemission, solving the long-standing inconsistency between measurements and theories, and demonstrating the contribution of coherent couplings with shakeup channels. The measurement of time delays gives unprecedented insight into the photoionization process, unraveling details of the atomic potential experienced by the escaping electron and capturing its dynamics.

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• Electron correlation calculations for atoms & ions
• Electronic excitation & ionization
• Single- and few-photon ionization & excitation
• Atoms
• Attosecond laser spectroscopy
• High-harmonic generation
• Photoionization
• Random phase approximation