Quasi-two-dimensional quantum states of H-2 in stage-2 Rb-intercalated graphite

Inelastic-incoherent-neutron scattering can be a valuable nanostructural probe of H-2-doped porous materials, provided the spectral peaks can be interpreted in terms of crystal-field-split hydrogen-molecule energy levels, which represent a signature of the local symmetry. Inelastic-neutron-scattering measurements as well as extensive theoretical analyses 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), with x = 0.8 or 1.0], a layered porous system with abundant spectral peaks, to assess whether the crystal-field-state picture enables a quantitative understanding of the observed structure. The experiments were made at 15 K on the QENS spectrometer at the intense pulsed neutron source. Potential-energy surfaces for molecular rotational and translational motion (parallel and perpendicular to the intercalant plane), as well 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 rotational 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 meV to a rotational-tunneling transition of an isotropic hindered-rotor oriented normal to the planes, but indicate a small azimuthal anisotropy and a lower barrier than for the isotropic case. A peak of low intensity at 4.0 meV is most likely a host feature, Based on the experimental isotope shifts and the theoretically predicted states, we conclude that spectral peaks at 11 and 22 meV are most likely related to center of mass excitations. We attribute the relatively weak peak at 32 meV to a librational excitation, and that at 44 meV to an out-of-plane vibration.