  We report on a Raman scattering investigation of the charge-density-wave
(CDW), quasi two-dimensional rare-earth tri-tellurides $R$Te$_3$ ($R$= La, Ce,
Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe$_3$ and CeTe$_3$ under
externally applied pressure. The observed phonon peaks can be ascribed to the
Raman active modes for both the undistorted as well as the distorted lattice in
the CDW state by means of a first principles calculation. The latter also
predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition.
The integrated intensity of the two most prominent modes scales as a
characteristic power of the CDW-gap amplitude upon compressing the lattice,
which provides clear evidence for the tight coupling between the CDW condensate
and the vibrational modes.
