Polycyclic aromatic hydrocarbons: negative ion formation following low energy (0–15 eV) electron impact

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Long-lived (autodetachment lifetimes > 10 −6 s) parent negative ions of a number of polycyclic aromatic hydrocarbons (M) have been observed when the electron affinity (EA) of M exceeds a few tenths of an electronvolt. For the larger hydrocarbons with EA (M) ⩾ 0.5 eV the long-lived parent negative ion was found to form via as many as three negative ion states above 0.0 eV. The production of long-lived parent negative ions at electron energies well above 0.0 eV and with much larger cross sections than at 0.0 eV is attributed to electron-excited Feshbach resonances (i.e., two-electron negative ion states). Results are also reported on the formation of (MH) − from M; (MH) − is the only dissociative attachment negative ion observed.