Some optical properties of graphite from IR to millimetric wavelengths

Abstract: Far infrared(FIR) data on the optical properties of graphite are presently lacking. An important step towards filling this gap was taken by Kuzmenko et al. (2008) who measured, on HOPG (Highly Oriented Pyrolitic Graphite) at normal incidence and from 10 to 300 K, the in-plane dielectric functions from 0.3 to 200 mum, and the reflectance between 0.3 and about 300 mum. We show here how, using recent developments of the electron theory of graphene, extended to graphite, it is possible to properly extrapolate the data farther even than 1000 mum, in effect all the way to Direct Current. The plasma frequency as well as the scattering rate of free electrons are shown to both decrease with T, but level off near 0 K, in agreement with theory. Along the way, we noticed significant discrepancies with the well-known and often used derivation of Philipp (1977) at room temperature, and also with previous data on temperature dependence and absorbance of graphitic material samples in different physical forms. Possible reasons for these discrepancies are discussed. Finally, the absorption efficiency of small graphitic spheres is deduced for the spectral range from 0.3 to 10000 mum. This may contribute to the discussion on model dust candidates for recently observed astronomical far infrared emissions.