Knowledge Bank

High resolution electron transmission $spectroscopy1,2$ is employed to study the low-lying temporary anions of ethylene, benzene, and their fluorine derivatives. In ethylene and benzene the ground state anions result from the occupation of the $b3g$ and $e2u\Pi \ast$ orbitals, respectively. Both of these anions are sufficiently long-lived to display structure due to nuclear motion. However, the ground state anions of fluoroethylenes and some of the flourobenzenes do not display fine structure due to nuclear motion indicating that they are subject to electron autodetachment in time much less than a vibrational period. Fluorine substitution is found to destabilize the ethylene anion. For example, vertical $\pi$ electron affinities of $-1.78$ and $-3.00$ eV are determined for ethylene and tetrafluoroethylene, respectively. This is surprising since fluorine substitution has little effect on the $\pi$ ionization potential. The first $\pi \ast$ orbitals of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene are degenerate, while this degeneracy is lifted in the other fluorobenzenes. The ground state anions of the fluorobenzenes are energetically more stable than that of benzene. The shifts and splittings in the $\pi \ast$ and $\pi$ orbitals of this series of compounds is explained in terms of a simple model including resonance and inductive interactions.