Optical Transmission of Graphite Compounds

Lamellar compounds of graphite are often, but not always, deeply colored. To study this color, a number of different compounds have been prepared from very thin flakes cleaved from annealed single crystals. The spectral transmission of these samples has been measured between 0.2 μ and either 2.2 μ or, in a few cases, 25 μ. All lamellar compounds exhibit a transmission ``window'' of half-width about 0.2 μ, in which the absorption coefficient is three to five times lower than that of graphite. The frequency of the transmission maximum is closely proportional to the square root of the density of holes or electrons in the graphite part of the compound, suggesting that it is the plasma oscillation of the excess electrons or holes which is responsible for the transmission. The effective mass of these carriers is close to unity. In addition to these transmission maxima, a related broad absorption is observed in the far infrared. These optical measurements permit rapid evaluation of ionization efficiencies in concentrated lamellar compounds, and facilitate kinetic studies of reaction rates and diffusion.