QUASI-2-DIMENSIONAL QUANTUM STATES OF H-2 IN STAGE-2 RB-INTERCALATED GRAPHITE

Inelastic-incoherent-neutron scattering can be a valuable nanostructur al probe of H-2-doped porous materials, provided the spectral peaks ca n be interpreted in terms of crystal-field-split hydrogen-molecule ene rgy levels, which represent a signature of the local symmetry. Inelast ic-neutron-scattering measurements as well as extensive theoretical an alyses have been performed on stage-2 Rb-intercalated graphite (Rb-GIC ), with phys isorbed H-2, HD, and D-2 [composition C(24)M(H-2)(x), wit h x = 0.8 or 1.0], a layered porous system with abundant spectral peak s, to assess whether the crystal-field-state picture enables a quantit ative understanding of the observed structure. The experiments were ma de at 15 K on the QENS spectrometer at the intense pulsed neutron sour ce. Potential-energy surfaces for molecular rotational and translation al motion (parallel and perpendicular to the intercalant plane), as we ll as the intermolecular interactions of hydrogen molecules in Rb-GIC, were calculated within local-density-functional theory (LDFT). A root 7 x root 7 periodic unit cell (with composition C28Rb) was treated in the calculations. Model potentials, parametrized using results of the LDFT calculations, were employed in schematic calculations of rotatio nal and translational excited state spectra of a single physisorbed H- 2 molecule in Rb-GIC. Results of our analysis are basically consistent with the assignment by Stead et al. of the lowest-lying peak at 1.4 m eV to a rotational-tunneling transition of an isotropic hindered-rotor oriented normal to the planes, but indicate a small azimuthal anisotr opy and a lower barrier than for the isotropic case. A peak of low int ensity at 4.0 meV is most likely a host feature, Based on the experime ntal isotope shifts and the theoretically predicted states, we conclud e that spectral peaks at 11 and 22 meV are most likely related to cent er of mass excitations. We attribute the relatively weak peak at 32 me V to a librational excitation, and that at 44 meV to an out-of-plane v ibration.