Structure and photoluminescence of helium-intercalated fullerite C-60

The intercalation of C-60 single crystals with helium is studied by powder x-ray diffractometry. It is established that the intercalation is a two-stage process: octahedral cavities are filled first and then tetrahedral ones, the chemical pressure being negative during both stages. The low-temperature (5 K) photoluminescence spectra of helium-intercalated fullerite C-60 are studied for the first time. The presence of helium in lattice voids is shown to reduce that part of the luminescent intensity which is due to the emission of covalently bound pairs of C-60 molecules, the so-called "deep traps" with the 0-0 transition energy close to 1.69 eV. The mechanism of the effect of intercalation with helium on the pair formation in fullerite C-60 is discussed