Quasi-two-dimensional quantum states ofH2in stage-2 Rb-intercalated graphite

Abstract

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-GIC͒, with physisorbed H 2 , HD, and D 2 ͓composition C 24 M͑H 2 ) x , with xϭ 0.8 or 1.0͔, 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 plane͒, as well as the intermolecular interactions of hydrogen molecules in Rb-GIC, were calculated within local-density-functional theory ͑LDFT͒. A ͱ7ϫͱ7 periodic unit cell ͑with composition C 28 Rb͒ 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.

Rotor in a cage: Infrared spectroscopy of an endohedral hydrogen-fullerene complex

“…H 2 @C 60 is different from quantum rotors studied so far, which were two-dimensional or showed hindered rotation, like H 2 on a Cu surface, 4 H 2 in intercalated graphite, 5 or C 2 in a metallofullerene. 6 The small mass and large rotational constant of H 2 make it the least sensitive of molecules to the corrugations of the potential surface.…”

Electron-volt neutron spectroscopy: beyond fundamental systems

“…The use of crystallography to study the arrangement of H 2 in GICs has proven elusive to date [188]. As alternative, H 2 uptake by RbC 24 [191] and CsC 24 [192] has been investigated using INS. The observed spectroscopic features at low energy transfers were attributed to the presence of two distinct adsorption sites.…”

Enhanced Hydrogen Dipole Physisorption, Final Report

“…As illustrated in Figure 37, they contain locally commensurate R19.11° patches surrounded by domain walls. The idealized structure (see Figure 36b) has been the basis of most ab initio studies of the H2/KC24 system 103,104 . Unfortunately the structure has a KC28 stoichiometry.…”