SINGLE-IONIZATION AND DOUBLE-IONIZATION POTENTIALS OF POLYCYCLIC AROMATIC-HYDROCARBONS AND FULLERENES BY PHOTON AND ELECTRON-IMPACT

Single- and double-ionization appearance potentials of 21 polycyclic aromatic hydrocarbons (PAHs) were determined by measurement of photoion yield curves using a synchrotron radiation source. Analogous measurements were made by electron impact on three PAHs. The origin of differences, of up to 4 eV, between appearance potentials for double ionization by photon and electron impact is discussed. The ratio of the double- to single-ionization potentials of PAHs was found to be 2.65 +/- 0.06 for photon impact, significantly lower than for electron-impact and charge-stripping measurements. An electrostatic model, which considers that PAHs ionize like fragments of graphite, is applied to the PAHs studied and to data on the fullerenes C-60 and C-70. The validity and limitations of the model are discussed and its analytical and predictive propensities concerning multiple-ionization potentials and processes are demonstrated.