Collisional dissociation of salt-cluster dianions

The structure of sodium chloride salt-cluster dianions of the type NanClm2- (n=7, 15, m=n+2) is studied using collisional dissociation with static argon gas targets. Collisions leading to single electron detachment leaving NanClm- are not observed for energies up to 400eV (Lab). An argument is proffered that the NanClm- ion is unstable toward dissociation. The loss of Cl- is only observed at relatively high energies (∼400eV), well above the expected threshold. This cross section is also observed to be small compared to that of molecular ''fission''. Dissociation (fission) into singly-charged conjugate ion-pairs, e.g., NaxClx+1-+NayCly+1- (x+y=n) occurs at energies below ∼1eV. Calculations show that the Na7Cl92- dianions are metastable as a result of a Coulomb barrier inhibiting dissociation. Experimentally estimated lifetimes against metastable decay are greater than 10μsec and are consistent with a long tunneling time through the Coulomb barrier. Quantum chemical calculations establish that the Coulomb barrier leading to dissociation into singly-charged anions, e.g. Na7Cl92-->NaCl2-+Na6Cl7-, is ∼0.8eV and is consistent with the observed thresholds