\textit{Ab initio} investigations of formation of the poly-bromine anions encapsulated inside the carbon nanotube.

We have performed \textbf{\textit{ab initio}} density-functional calculations to investigate the electronic and geometric structure of the bromine adsorbates inside the carbon nanotube. It is found that the charged odd-membered molecular species (Br$_{3}$ or Br$_{5}$ ) are energetically favored inside the carbon nanotube rather than common Br$_{2}$ molecule. Vapor phase of bromine molecules (Br$_{2})$ could exothermically adsorb into the nanotubes, and in turn, transform into the Br$_{3}$ or Br$_{5}$ structures without a significant energy barrier. Such a formation of the poly-bromine anions accompanies a strong charge transfer from the nanotube to the adsorbates, rendering the encapsulating nanotube strongly hole-doped. We suggest that an exposure of the tip-opened carbon nanotube samples to a modest Br$_{2 }$partial pressure could result in strong hole-doped, and thus nearly metallic nanotube samples.