Role of electron-electron and electron-phonon interaction effect in the   optical conductivity of VO2
  We have investigated the charge dynamics of VO2 by optical reflectivity measurements. Optical conductivity clearly shows a metal-insulator transition. In the metallic phase, a broad Drude-like structure is observed. On the other hand, in the insulating phase, a broad peak structure around 1.3 eV is observed. It is found that this broad structure observed in the insulating phase shows a temperature dependence. We attribute this to the electron-phonon interaction as in the photoemission spectra. 
K. Okazaki, S. Sugai, Y. Muraoka, Z. Hiroi