  We present optical spectroscopy measurements on rare-earth ditelluride single
crystals of LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16). The
measurements reveal formation of charge density wave energy gaps at rather high
energy levels, e.g. 2$\Delta\sim$ 8500 \cm for LaTe$_{1.95}$, and 6800 \cm for
CeTe$_{1.95}$. More strikingly, the study reveals that, different from the
rare-earth tri-tellurides, the Te vacancies and disorder effect play a key role
in the low-energy charge excitations of ditelluride systems. Although an
eminent peak is observed between 800 and 1500 \cm in conductivity spectra for
LaTe$_{1.95}$, and CeTe$_{1.95-x}$Se$_x$ (x=0. 0.16), our analysis indicates
that it could not be attributed to the formation of a small energy gap, instead
it could be well accounted for by the localization modified Drude model. Our
study also indicates that the low-tempreature optical spectroscopic features
are distinctly different from a semiconducting CDW state with entirely gapped
Fermi surfaces.
