Photoelectron imaging of doubly charged anions, (-)O2C(CH2)nCO2(-) (n = 2-8): observation of near 0 eV electrons due to secondary dissociative autodetachment.

The hallmark of multiply charged anions is the repulsive Coulomb barrier (RCB), which prevents low-energy electrons from being emitted in photodetachment experiments. However, using photoelectron imaging, we have observed persistent near 0 eV electrons during photodetachment of doubly charged dicarboxylate anions, (-)O(2)C(CH(2))(n)CO(2)(-) (D(n)(2-), n = 2-8). Here we show that these low-energy electron signals are well structured and are independent of the detachment photon fluxes or energies. The relative intensities of these signals are dependent on n, with maxima at n = 2, 4, and 6. These near 0 eV electrons cannot come from direct photodetachment of the dianions and are proposed to come from decarboxylation of the product radical anions upon photodetachment of the parent dianions [(*)O(2)C(CH(2))(n)CO(2)(-) --> CO(2) + (*)(CH(2))(n)CO(2)(-)], followed by dissociative autodetachment [(*)(CH(2))(n)CO(2)(-) --> (CH(2))(n) + CO(2) + e] or hydrogen-transfer-induced electron detachment [(*)(CH(2))(n)CO(2)(-) --> CH(2)=CH(CH(2))(n-2)CO(2)H + e]. Energetic considerations suggest that these processes are exothermic. It is further observed that solvation by one water molecule quenches the low-energy electron signals in the spectra of D(n)(2-)(H(2)O), consistent with the proposed mechanisms. These indirect dissociative autodetachment processes are expected to involve cyclic transition states for n > 2, which is in agreement with the dependence on the chain length due to the anticipated strains in the intermediate steps. The quenching of the low-energy electron signals by one water molecule demonstrates the importance of solvation on chemical reactions.