Fig. 1.

Illustration of SFRS and pump-probe HHG. (A) Schematic diagram of the multipulse HHG measurement. (B) Sketch of the SPRS signal for detecting the electronic coherences. Coherence between excited states is prepared by SPRS using pulses $P_1$ and $P_2$. $P_3$ is used to excite the system before spontaneous emission of the detected photon, which carries information about coherence. (C) Sketch of the time-domain HHG spectroscopy. Coherent excitation is prepared by a recombination of the ionized electrons, followed by a probe that is used to generate HHG signal. Dependence of the HHG on the time delay between pulses contains information about the electron dynamics. (D) Double-sided Feynman diagram for the first-order response involving two pulses (see SI Appendix, section C, for more details of diagram rules).