The measurement of intercalant vibrations using Raman and infrared spectroscopies

It is difficult to observe intercalant vibrations in graphite intercalation compounds (GICs) using Raman and infrared reflection spectroscopies because GICs have a very high absorption coefficient in the visible and are highly reflective in the infrared. We present here methods that overcome these difficulties. These methods use the very large anisotropy in the optical properties of these systems. Raman spectra can be successfully measured by using the selection rule which approximately forbids optically induced band transitions with light polarized in the 'c' direction. Both direct and phonon assisted transitions are almost totally forbidden. Consequently the penetration depth of the incident as well as the scattered light is largely enhanced leading to an observable Raman scattering spectrum. In the infrared we show that the reflectivity of light from an interface between a GIC sample and an infrared transparent material is qualitatively different from the reflectivity at an air GIC interface, provided that the optical dielectric constant of the IR transparent material is larger than the perpendicular component of the GIC optical dielectric tensor. It is further shown both theoretically and experimentally that the reflectivity in the first case is highly sensitive to phonons which are IR active to light polarized in the 'c' direction. The IR reflectivity spectra of two GICs are presented illustrating their sensitivity to phonons. Furthermore we show that the line shapes observed are consistent with our theoretical pedictions.