  Polarized Raman scattering has been performed on CaC6 superconductor. We
identify two of the three Raman active Eg phonon modes at 440 and 1508 cm-1
expected for the R-3m space group of CaC6. These first order scattering modes
appear along with the D and G bands around 1300 cm-1 and 1600 cm-1 that are
similar in origin to the corresponding bands in plain graphite. The intensities
of the D and G bands in CaC6 correlate with degree of disorder. The D band
arises from the double resonant Raman scattering process; its frequency shifts
as a function of excitation energy with approximately 35 cm-1/eV. The double
resonant Raman scattering probes phonon excitations with finite wave vector q.
We estimate the characteristic spacing of structural defects to be on the scale
of about 100 Angstrom by comparing the intensity of the D band and the 1508
cm-1 Eg mode in CaC6 to calibrated intensity ratio of analogous bands in
disordered graphites. A sharp superconducting coherence peak at 24 cm-1 is
observed below Tc.
