The wave vector dependence of coupled plasmon-LO phonon modes is studied with opaque Raman spectroscopy. The dispersion of the coupled modes is directly related to the wavevector dependence of the dielectric function of the electron gas and is influenced by finite temperatures and finite damping. The broadening of the wavevector due to absorption is also important. These effects are explained with the quantum mechanical Lindhard dielectric function for finite temperature including electron scattering in the relaxation time approximation of Mermin.