  In monolayer semiconductor transition metal dichalcogenides, the
exciton-phonon interaction is expected to strongly affect the photocarrier
dynamics. Here, we report on an unusual oscillatory enhancement of the neutral
exciton photoluminescence with the excitation laser frequency in monolayer
MoSe2. The frequency of oscillation matches that of the M-point longitudinal
acoustic phonon, LA(M). Oscillatory behavior is also observed in the
steady-state emission linewidth and in timeresolved photoluminescence
excitation data, which reveals variation with excitation energy in the exciton
lifetime. These results clearly expose the key role played by phonons in the
exciton formation and relaxation dynamics of two-dimensional van der Waals
semiconductors.
