Single- and double-ionization potentials of polycyclic aromatic hydrocarbons and fullerenes by photon and electron impact

Abstract

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 C60 and C70. The validity and limitations of the model are discussed and its analytical and predictive propensities concerning multiple-ionization potentials and processes are demonstrated.