Optical probes of the lattice dynamics of graphite

First and second order Raman and first order infra-red reflectance spectra of highly oriented pyrolytic graphite (HOPG) have been obtained for both E∥c and Efc. From the infra-red measurement, the A_2u out of plane mode is identified for the first time and occurs at 868 cm^-1. The macroscopic effective changes of both the A_2u and E_1u modes have also been calculated. The second order Raman spectra have been recorded for graphite forms of various crystallite sizes. The second order Raman spectra of both HOPG and single crystal graphite exhibit an anomalously sharp feature at 3 248 cm^-1, which is, in fact, sharper than the first order zone center E_2g band at 1 581 cm^-1. Of the six recent, and in some cases mutually inconsistent, lattice dynamics calculations on graphite, only that due to Nicklow et al. is compatible with the 3 248 cm^-1 feature, and even that model does not provide a satisfactory explanation of the entire second order spectrum.