To understand the tr-ESFG dynamics of pristine monolayer MoS 2 and Ru-MoS 2 , it is necessary to dive into the theoretical framework of tr-ESFG, which, as shown below, has different spectral features than the more commonly applied TA spectroscopy. All signals are described using the double-side Feynman diagram that can be expanded into full response functions to calculate the spectra. In the Feynman diagram, the straight arrows describe the light-matter interactions, while the arrows on the left (right) hand sides indicate interactions to the ket (bra) states in the Dirac notation. The inward (outward) arrows correspond to the creation (annihilation) of a quantum state. For example, Figure describes ground state bleach (GSB), in which the systems are brought back to the ground state after the first two visible light interactions and are subsequently probed by the ESFG pulse sequence on the left-hand side. Similarly, Figure describes the stimulated emission (SE) in which the systems is in an excited state after the first two interactions and is then further stimulated emitting SFG signals. Lastly, in Figure , the excited states interact with the ESFG pulse sequence to create higher excited state coherences, akin to excited state absorption (ESA). These processes are similar to TA spectroscopy.