We study the nonlinear optical response of argon to a four-wave-mixing pulse sequence consisting of an extreme-ultraviolet pulse, a collinear IR dressing pulse, and a delayed noncollinear IR probe pulse. Sequential absorption of an extreme ultraviolet photon and an IR photon from the collinear beams excites the $3s^{-1}4p$ bright state and the $3s^{-1}3d/5s$ dark states. The subsequent absorption of an IR photon from the noncollinear beam then leads to an angled extreme ultraviolet emission, whose variation with delay encodes the coupling between autoionizing states, the lifetimes of dark states, and nonperturbative effects. Both our measurements and ab initio simulations of the angled four-wave-mixing signal reveal a double-peak structure in the delay dependence, in good agreement with each other. We attribute the central minimum between the two peaks to rapid Rabi cycling, driven by the collinear IR dressing pulse, between the dark states and the $3s^{-1}4p$ resonance, which leads to destructive interference in the final transition amplitude.

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• Autoionization & Auger processes
• Optical transient phenomena
• Strong-field-induced spectra
• Ultrafast phenomena
• Attosecond laser spectroscopy
• First-principles calculations
• Four-wave mixing
• Hartree-Fock methods
• Transient absorption spectroscopy
• Ultrafast pump-probe spectroscopy