Hydrogen site occupancies in single-walled carbon nanotubes studied by inelastic neutron scattering

Neutron inelastic scattering spectra from high quality single-wall carbon nanotubes (SWNTs), progressively dosed with hydrogen, have been measured with high resolution at different temperatures in the vicinity of 20 K. The spectra consist of two parts. Firstly, a relatively sharp complex peak, due to scattering from translationally bound molecules undergoing rotational transitions, is observed at around 14.5 meV. Secondly, there is a series of broad features at higher energies originating from roto-vibrational transitions corresponding to J = 0 to 1 rotational change plus n = 0 to n = 1, 2, etc molecular centre-of-mass vibrational transitions, as well as from molecule centre-of-mass recoil. The structure of the complex sharp peak suggests that the molecular hydrogen is adsorbed on at least two different adsorption sites on the surfaces of the nanotube bundles.