Dianion formation from anion-alkali metal charge exchange reactions: TCNQ$^{-}$ + Na $\to $ TCNQ$^{--}$ + Na$^{+}$

The interaction of an electron with an anion is characterized by a long-range coulomb repulsion and a short range polarizability attraction giving rise to a coulomb barrier. The permanent addition of an extra electron to a negatively charged anion requires tunneling through the barrier or attachment of the electron over the top of this coulomb barrier followed by disposal of the excess energy. Charge-exchange collisions of an anion with an alkali atom utilize the latter channel to produce permanent dianions with cross sections of $\sim $1 {\AA}$^{2}$. We have previously examined the reaction TCNQ-F$_{4}^{-}$ + Xe $\to $ TCNQ-F$_{4}^{--}$ + Xe$^{+}$ and reported a delayed threshold and quantum phase interference effects in the charge exchange cross section.\footnote{S. Yu. Ovchinnikov, et al. Phys. Rev. A, 73, 64704(2006)} Employing sodium as the collision partner, the cross section is seen to increase with decreasing energy with a threshold below 180 eV (com). A new apparatus has been constructed to allow measurements down to energies below the expected threshold ($\sim $41 eV, laboratory energy based upon a 1 eV second electron affinity). This method has been used to study the reaction TCNQ$^{-}$ + NA $\to $ TCNQ$^{--}$ + Na$^{+}$ and will provide one of the first measurements of second electron affinities for molecular anions.