College Park, Maryland      June 6 - 10 , 2004

M4-C4 (4:45 PM): Molecular Rotation and Translation of Hydrogen Adsorbed on Planar Graphite and in Single-Wall Carbon Nanotubes

P.A. Georgiev, D.K. Ross, J. Fernandez-Garcia, I. Morrison (Institute for Materials Research, University of Salford, Salford, M5 4WT)

We present neutron inelastic scattering spectra measured at 17 K on the TOSCA spectrometer, ISIS, UK, from different amounts of para-Hydrogen within the sub- and multilayer region physisorbed on Grafoil and high quality Single-Wall Carbon Nanotubes. Our results show that on the graphite surface and for concentrations of up to the densest monolayer the rotational motion of the adsorbed molecules is unperturbed. A downshift of 0.2meV is attributed to the different Zero Point Energies (ZPE) of the Center-of-Mass (CoM) out of plane motion for the para- and ortho- molecular species, as well as, a small increase in the H-H bond length. On the contrary, a 1.5 meV splitting between the states within the J = 1 manifold is observed at low surface coverage, where due to the twice higher binding energy as compared to flat graphite, the molecules are preferentially adsorbed on the groove sites of the nanotube bundles. At higher surface coverages the scattering is dominated by freely rotating molecules adsorbed at the external convex tube surface and in higher molecular layers. Our new and unique high-resolution data from a few concentrations in the multilayer region shows for first time that due to the vertical solid-like behavior of the second and possibly even the third molecular layers, the J = 0 to J = 1 rotational line of hydrogen adsorbed on Grafoil is also split. The higher molecular layers show no ZPE effect on the rotational line. The 0 - 1 rotational transition due to molecules adsorbed between the graphite surface and higher molecular layers is now split. It is anticipated that for these molecules the mixing between the different m-states of the J = 1 level is now removed and a different rotational-translational coupling scheme operates.

The mean translational kinetic energies as functions of the surface concentration are derived from the high-energy part of each spectrum.

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