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