Observation of intravalley phonon scattering of 2s excitons in MoSe₂ and WSe₂ monolayers

We present a high-resolution resonance Raman study of hBN encapsulated MoSe₂ and WSe₂ monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe₂ A2s and MoSe₂ A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k = 0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe₂ A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex. (paper)