  We report optical reflectivity study on pure and boron-doped diamond films
grown by a hot-filament chemical vapor deposition method. The study reveals the
formation of an impurity band close to the top of the valence band upon
boron-doping. A schematic picture for the evolution of the electronic structure
with boron doping was drawn based on the experimental observation. The study
also reveals that the boron doping induces local lattice distortion, which
brings an infrared-forbidden phonon mode at 1330 cm$^{-1}$ activated in doped
sample. The antiresonance characteristic of the mode in conductivity spectrum
evidences the very strong coupling between electrons and this phonon mode.
