Space-dependent diffusion of hydrogen in carbon nanotubes

We analyse the dynamics of hydrogen molecules trapped in carbon nanotubes, with emphasis on the motion along the tube axis. In the configuration that we have used, the hydrogen molecules dispose themselves in a cylindrical shell around the tube axis. We correct some of our previous results [G. Garberoglio, R. Vallauri, Single particle dynamics of molecular hydrogen in carbon nanotubes, Phys. Lett. A 316 (6) (2003) 407], which suffer from finite size-effect and extend the analysis to the self-intermediate scattering function. We find that the self-intermediate scattering function reaches the free particle regime for wave vectors smaller than the one corresponding to the first diffraction maximum. This behaviour is shown to stem from the quasi two dimensional arrangement of the hydrogen molecules inside the carbon nanotubes.