Experimental Q-dependence of the rotational JÂ =Â 0-to-1 transition of molecular hydrogen adsorbed in single-wall carbon nanotube bundles

Inelastic neutron scattering spectra of para-Hydrogen adsorbed in single-wall carbon nanotubes have been measured at 20 K, at different surface loadings, and at a set of kinematic trajectories. These show the presence of at least two different adsorption sites in the nanotube bundle. Only a weak hindrance to rotation is observed on the more adsorptive site which is preferentially occupied at low H2 concentrations while a completely free rotation was found at the second and weaker site where the determined centre-of-mass dynamics suggests H2-H2 distances similar to those on the graphite surface and in the bulk solid.