BROMINATED FULLERENE CRYSTALS, C_(60)Br_8 AND C_(60)Br_(24) STUDIED THROUGH POSITRON ANNIHILATION

摘要

Doppler broadening spectra for brominated fullerene crystals, C_(60)Br_8 and C_(60)Br_(24), showed the full widths at half maximum (FWHM) of 2.9 and 3.0 keV, respectively, which are larger than the width for pristine C_(60), 2.8 keV. The wide spectra are explained in terms of a preferential annihilation of positrons with electrons in bromine atoms which effectively cover the surface of a C_(60) carbon skeleton. The lifetimes of positrons for C_(60)Br_8 and C_(60)Br_(24), 371 and 408 ps, respectively, are shorter than the lifetime for C_(60), 427 ps, which is attributed to an increase in annihilation rate on the molecular surface. When brominated fullerene crystals are irradiated with electrons in an electron microscope operated at 100 kV, debromination from these crystals occurs and C_(60) molecules thus generated are crystallized to the hep system.