From specific heat measurements in high quality H2@C60 samples performed over a broad temperature range, we obtain the smallest yet observed splitting of rotational energy sublevels of encapsulated single H2 molecules, 0.1-0.2 meV, in the nearly spherical potential well provided by highly isotropic C60 cages. Additionally, we find evidence of the quantized oscillation state of isolated H2 in the C60 cage. The minuscule splitting indicates that H2@C60 provides unprecedented opportunities to study free-molecule quantum dynamic properties.

We present an extensive soft x-ray photoelectron spectroscopy study on the core levels of the endohedral atoms of Ba 8 Ga 16 Ge 30 ͑BGG͒, Sr 8 Ga 16 Ge 30 ͑SGG͒, Eu 8 Ga 16 Ge 30 ͑EGG͒, and Sr 8 Ga 16 Si 30 ͑SGS͒ single crystals, performed at a high-energy facility. Unexpected features evidencing the different shift for the 2a and 6d sites between Ba 4d and Sr 3d are observed for BGG and SGG. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the surprising conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which is in contrast to the past consensus that BGG and SGG have the same framework structure. The differences are further supported by analysis of the Ga 3d core levels. This may suggest thorough reconsiderations on earlier interpretations of experimental data.

The electronic states of Ba24Ge100 are studied by soft x-ray photoelectron spectroscopy (XPS) at a high-energy photon factory. A large reduction in the density of states (DOS) at the Fermi level is clearly shown before and after the electronic phase transition at 200 K. The changes in the spectrum widths and the fine structures of the core-level Ba 4d spectra give a very reasonable indication of the Ba-atom rattlings in the clathrate polyhedra. On-resonance experiments using the excitation from Ba 3d to 4f levels display that the wave functions of Ba 5d and 6s orbitals give only a small contribution to make a Fermi surface through the hybridization with the Ge20 cluster orbitals. Importantly, reliable values of the DOS at the Fermi level NEF are successfully deduced, using two data sets of DOS obtained from high-resolution XPS and the total magnetic susceptibilities by a superconducting quantum interference device, to be 0.149 and 0.0427 states eV(-1) (Ge atom)(-1) for a high-temperature and for a low-temperature phase.

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