  We introduce an analysis model, an extended Drude-Lorentz model, and apply it
to Fe-pnictide systems to extract their electron-boson spectral density
functions (or correlation spectra). The extended Drude-Lorentz model consists
of an extended Drude mode for describing correlated charge carriers and Lorentz
modes for interband transitions. The extended Drude mode can be obtained by a
reverse process starting from the electron-boson spectral density function and
extending to the optical self-energy and, eventually, to the optical
conductivity. Using the extended Drude-Lorentz model, we obtained the
electron-boson spectral density functions of K-doped BaFe$_2$As$_2$ (Ba-122) at
four different doping levels. We discuss the doping-dependent properties of the
electron-boson spectral density function of K-doped Ba-122. We also can include
pseudogap effects in the model using this approach. Therefore, this approach is
very helpful for understanding and analyzing measured optical spectra of
strongly correlation electron systems, including high-temperature
superconductors (cuprates and Fe-pnictides).
