Dispersive Raman Mode of Layered Compound 2H-MoS₂ under the Resonant Condition

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A frequency shift of a Raman peak is observed in 2H-MoS₂, when the exciting laser frequency ω_i is tuned across the A and B excitonic levels. The frequency shift decreases as ω_i increases. It can be explained as due to a resonant two-phonon Raman process of the successive emission of a dispersive longitudinal quasi-acoustic phonon and a dispersionless E_1u² phonon both along the c axis. The translational exciton masses along the c axis are estimated to be (1.3±0.2)m₀ for the 1s state of the A exciton and (1.6±0.3)m₀ for the 1s state of the B exciton. The observed Raman cross section is consistent with qualitative estimation in terms of this Raman process.