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Raman scattering excitation (RSE) is an experimental technique in which thespectrum is made up by sweeping the excitation energy when the detection energyis fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on fourhigh quality monolayers (ML) of semiconducting transition metal dichalcogenides(S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated inhexagonal BN. The outgoing resonant conditions of Raman scattering reveal anextraordinary intensity enhancement of the phonon modes, which results inextremely rich RSE spectra. The obtained spectra are composed not only ofRaman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but theappearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised.The intensity profiles of the A$'_1$ modes in the investigated MLs resemble theemissions due to neutral excitons measured in the corresponding PL spectra forthe outgoing type of resonant Raman scattering conditions. Furthermore, for theWSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was inresonance with the neutral exciton line. The strength of the exciton-phononcoupling (EPC) in S-TMD MLs strongly depends on the type of their groundexcitonic state, $i.e.$ bright or dark, resulting in different shapes of theRSE spectra. Our results demonstrate that RSE spectroscopy is a powerfultechnique for studying EPC in S-TMD MLs.

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides