Raman scattering with infrared excitation resonant with the MoSe2 indirect band gap

Resonance Raman scattering, which probes electrons, phonons, and their interplay in crystals, is extensively used in two-dimensional materials. Here we investigate Raman modes in MoSe2 at different laser excitation energies from 2.33 eV down to the near infrared 1.16 eV. The Raman spectrum at 1.16 eV excitation energy shows that the intensity of high-order modes is strongly enhanced if compared to the first-order phonon modes' intensity due to resonance effects with the MoSe2 indirect band gap. By comparing the experimental results with the two-phonon density of states calculated with density functional theory, we show that the high-order modes originate mostly from two-phonon modes with opposite momenta. In particular, we identify the momenta of the phonon modes that couple strongly with the electrons to produce the resonance process at 1.16 eV, while we verify that at 2.33 eV the two-phonon modes' line shape compares well with the two-phonon density of states calculated over the entire Brillouin zone. We also show that by lowering the crystal temperature, we actively suppress the intensity of the resonant two-phonon modes and we interpret this as the result of the increase of the indirect band gap at low temperature that moves our excitation energy out of the resonance condition.