Analysis of infrared optical transitions in graphite

We report new detailed measurements of reflectivity and thermoreflectivity (TR) spectra of graphite in the 0.04-1.6-eV region. From experimental data, the thermoderivative line shapes $\Delta {\epsilon }_2$ for both 0.74-eV ($A_1$) and 0.88-eV ($A_2$) TR structures, associated with two nearly degenerate interband transitions near the $K$ point in the Brillouin zone, are obtained. On the basis of the Slonczewski-Weiss (SW) model, the theoretical ${\epsilon }_2$ function and the corresponding change $\Delta {\epsilon }_2$ induced by temperature modulation are calculated. The shift, broadening, and Fermi occupation effects are taken into account. Good agreement between experimental and theoretical results is obtained, supporting the transition assignment. Only the broadening effect appears to be important in the experimental modulation spectrum. Considerations on the SW model parameters are made.