DIRECT DOUBLE IONIZATION OF C-60 AND C-70 FULLERENES USING SYNCHROTRON RADIATION

The double photoionization threshold region of C60 and C70 has been studied using single-photon ionization with synchrotron radiation. Mass selection is realized with a reflectron time-of-flight mass spectrometer (RETOF). The ionization potentials for double ionization of C60 and C70 have been determined to be 19.00+/-0.03 and 18.84+/-0.03 eV, respectively. In the 6 eV range above the IP covered in this report, the doubly charged ion yield spectra exhibit no significant structure and rise continuously and nearly linearly as a function of photon energy. A change of slope occurs in the C-60(2+) signal 1.3 eV above threshold which could be due to participation of an excited C-60(2+) state.