  Excitonic resonance and binding energies can be altered by controlling the
environmental screening of the attractive Coulomb potential. Although this
screening response is often assumed to be static, the time evolution of the
excitonic quasiparticles manifests a frequency-dependence in its Coulomb
screening efficacy. In this letter, we investigate a ground (1s) and first
excited exciton state (2s) in a multi-layered transition metal dichalcogenide
(MoS$_2$) upon ultrafast photo-excitation. We explore the dynamic screening
effects on the latter and show its resonance frequency is the relevant
frequency at which screening from the smaller-sized 1s counterparts is
effective. Our finding sheds light on new avenues of external tuning on
excitonic properties.
