  Electron correlations play a dominant role in the charge dynamics of the
cuprates. We use resonant inelastic x-ray scattering (RIXS) to track the doping
dependence of the collective charge excitations in electron doped
La$_{2-x}$Ce$_{x}$CuO$_{4}$(LCCO). From the resonant energy dependence and the
out-of-plane momentum dependence, the charge excitations are identified as
three-dimensional (3D) plasmons, which reflect the nature of the electronic
structure and Coulomb repulsion on both short and long length scales. With
increasing electron doping, the plasmon excitations show monotonic hardening in
energy, a consequence of the electron correlation effect on electron structure
near the Fermi surface (FS). Importantly, the plasmon excitations evolve from a
broad feature into a well defined peak with much increased life time, revealing
the evolution of the electrons from incoherent states to coherent
quasi-particles near the FS. Such evolution marks the reduction of the
short-range electronic correlation, and thus the softening of the Mottness of
the system with increasing electron doping.
