Collisional dissociation of salt-cluster dianions

Abstract

The structure of sodium chloride salt-cluster dianions of the type Na n Cl m 2 - ( n = 7, 15, m = n + 2) is studied using collisional dissociation with static argon gas targets. Collisions leading to single electron detachment leaving Na n Cl m - are not observed for energies up to 400 eV (Lab). An argument is proffered that the Na n Cl m - ion is unstable toward dissociation. The loss of Cl − is only observed at relatively high energies (∼400 eV), 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., Na x Cl x + 1 - + Na y Cl y + 1 - ( x + y = n) occurs at energies below ∼1 eV. Calculations show that the Na 7 Cl 9 2 - dianions are metastable as a result of a Coulomb barrier inhibiting dissociation. Experimentally estimated lifetimes against metastable decay are greater than 10 μsec 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. Na 7 Cl 9 2 - → NaCl 2 - + Na 6 Cl 7 - , is ∼0.8 eV and is consistent with the observed thresholds.