  We carried out room-temperature far-infrared (40--650~cm$^{-1}$) transmission
measurements on undoped and bromine-doped powder samples of carbon nanofibers
in stacked-cup cone geometry. The transmission spectra of both doped and
undoped samples were fit to a Drude-Lorentz model. A single Drude component and
a small bandgap (around 8~meV) semiconducting Lorentzian component along with 3
other Lorentz components were essential to get a good fit in the entire
measured frequency range. A decreased metallic conductivity along with a
red-shift of the lowest semiconducting gap was found after bromine doping. A
significant decrease in the scattering rate upon heavy doping has been
qualitatively explained as partial ordering of intercalated dopant ions.
Absorption spectra were derived from the transmission spectra under the
assumption of non-dispersive reflectance. These spectra were compared to
Drude-Lorentz model absorption spectra. The free-carrier density of the n-type
powder and the electronic mean free path were estimated and compared with
previously reported values for single-walled nanotubes and pyrolytic graphite.
