Momentum distribution of helium and hydrogen in nanotubes

We compute the momentum distribution of helium atoms and hydrogen molecules absorbed within an ordered bundle of carbon nanotubes. The results vary significantly as a function of coverage and manifest the strong anisotropy and localization of this geometry. For example, the root-mean-square momentum component perpendicular to the bundle axis can be about three times larger for interstitial molecules than for molecules moving in an axial phase confined by a cylindrical film of particles coating the tube's inner wall. These results (which are consequences of the uncertainty principle) indicate that the momentum distribution is a useful signature of the local geometry and quantum state of the absorbed particles in nanotube bundles.