Fig. 1. Experimental scheme.

Intense twin-seed pulses are generated in an ultra-stable monolithic interferometer. The pulse delay τ is set by a wedge-based delay line (DL), and phase-locked acousto-optic modulators (AOMs) control the relative phase ϕ₂₁ of the seed pulses. The time- and phase-controlled pulse pairs seed the high-gain harmonic generation (HGHG) process in the FEL, resulting in coherent XUV pulse pairs with precisely controlled timing and relative phase. The XUV pulse pair tracks the real-time evolution of electronic coherences induced in an atomic beam sample. Detection is done via photoionization with a third pulse from a near infrared (NIR) laser. Both photoelectrons and -ions are detected with a combined magnetic bottle electron (MB) and ion time-of-flight (iTOF) spectrometer. A continuous-wave reference laser is used to trace the phase evolution and jitter in the interferometer, recorded with a photodiode (PD). This signal is used for rotating frame sampling and phase-sensitive detection of the mass/energy-gated ion/electron yields with a lock-in amplifier (LIA).