  Raman investigation of MoSe2 was carried out with eight different excitation
energies. Seven peaks, including E1g, A1g, E2g1, and A2u2 peaks are observed in
the range of 100-400 cm-1. The phonon modes are assigned by comparing the peak
positions with theoretical calculations. The intensities of the peaks are
enhanced at different excitation energies through resonance with different
optical transitions. The A1g mode is enhanced at 1.58 and 3.82 eV, which are
near the A exciton energy and the band-to-band transition between higher energy
bands, respectively. The E2g1 mode is strongly enhanced with respect to the A1g
mode for the 2.71- and 2.81-eV excitations, which are close to the C exciton
energy. The different enhancements of the A1g and E2g1 modes are explained in
terms of the symmetries of the exciton states and the exciton-phonon coupling.
Other smaller peaks including E1g and A2u2 are forbidden but appear due to the
resonance effect near optical transition energies.
