Multilayers of fullerenes with and without endohedral Ar units, Ar@C60 and C60, were investigated by photoemission and density-functional theory. The stoichiometry and the endohedral nature of Ar were checked by x-ray photoelectron spectroscopy and x-ray photoelectron diffraction. Valence-band ultraviolet photoemission spectra showed a strong hybridization of the Ar 3p valence shell with the 6T1u molecular orbital of C60. A hybridization gap of 1.6 ± 0.2 eV was found. This is in agreement with density-functional theory, which predicts 1.47 eV and indicates that Ar@C60 is a noble gas compound with a strong coupling between Ar and the C60 cage. No giant Ar photoemission cross section as previously predicted for the gas phase in Phys. Rev. Lett. 99 243003 (2007) was found.
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