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, -O2C(CH2)(n)CO2- (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 [(O2C)-O-center dot(CH2)(n)CO2- -> CO2 + (center dot)(CH2)(n)CO2-], followed by dissociative autodetachment [(center dot)(CH2)(n)CO2- -> (CH2)(n) + CO2 + e] or hydrogen-transfer-induced electron detachment [(center dot)(CH2)(n)CO2- -> CH2=CH(CH2)(n-2)CO2H + 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-)(H2O), 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 salvation on chemical reactions.