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 $\mathrm{MoSe}_2$ 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 $\mathrm{MoSe}_2$ 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.

中文翻译：

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红外激发拉曼散射与 MoSe2 间接带隙共振

共振拉曼散射探测电子、声子及其在晶体中的相互作用，广泛用于二维材料。在这里，我们研究拉曼模式$\mathrm{钼硒}_2$在从 2.33 eV 到近红外 1.16 eV 的不同激光激发能量下。激发能量为 1.16 eV 的拉曼光谱表明，与一级声子模式的强度相比，高阶模式的强度由于共振效应而大大增强$\mathrm{钼硒}_2$间接带隙。通过将实验结果与密度泛函理论计算的双声子状态密度进行比较，我们表明高阶模式主要来自具有相反动量的双声子模式。特别是，我们确定了与电子强烈耦合以在 1.16 eV 处产生共振过程的声子模式的动量，同时我们验证了在 2.33 eV 时，双声子模式的线形与在整个布里渊区计算的状态。我们还表明，通过降低晶体温度，我们主动抑制了共振双声子模式的强度，我们将其解释为低温下间接带隙增加的结果，这将我们的激发能量移出了共振条件.