POLYCYCLIC AROMATIC-HYDROCARBONS - NEGATIVE-ION FORMATION FOLLOWING LOW-ENERGY (0-15 EV) ELECTRON-IMPACT

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) greater-than-or-equal-to 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.