Intercalant vibrations in a graphite intercalation compound observed by infrared reflectivity at a graphite-germanium interface

We report on a new method to observe intercalant vibrations in graphite intercalation compounds (GIC) using ir reflectivity. It is shown that, under certain conditions, phonons affect the reflectivity, at an interface between an ir-transparent material (IRTM) and an intercalated graphite sample, much more than at an air-sample interface. The results of reflectivity measurements as a function of the angle of incidence support the theoretical analysis and provide the dielectric tensors for pure highly oriented pyrolytic graphite and for stage-1 C/AsF5 GIC samples. Using these results we calculate the relative sensitivity of reflectivity spectra to phonon contributions as a function of the angle of incidence. We show that the sensitivity has a maximum at an angle c for which the electromagnetic radiation in the graphite propagates parallel to the graphite surface. We also present insitu measurements of the reflectivity as a function of photon energy. The spectra display the evolution of the A2u mode at different intercalation times as well as five lines which correspond to vibrations of AsF3, AsF5, AsF6-, and possibly As2F11-. The line shapes are also clearly explained.