Internal energy deposition in electron capture dissociation measured using hydrated divalent metal ions as nanocalorimeters

Extensively hydrated divalent metal ions are used as nanocalorimeters to measure the internal energy deposition resulting from electron capture. For M(H2O)(32)(2+), M = Mg, Ca, Sr, and Ba, two dissociation pathways are observed: loss of a water molecule from the precursor (similar to 6%) owing to activation from blackbody photons or collisions with residual background gas, and loss of between 9 and 11 water molecules from the reduced precursor formed by electron capture. The binding energy of a water molecule to the reduced precursor ions is estimated to be approximately 10 kcal/mol. From the distribution of water molecules lost, corrected for residual activation, the average and maximum internal energy deposited into these ions is determined to be similar to 4.4 and similar to 4.8-5.2 eV, respectively. The average internal energy deposition does not depend significantly on metal ion size (similar to 0.1 eV difference for Mg to Ba) despite the large (5.0 eV) difference in second ionization energies for the isolated atoms. Similar results were obtained for Ca(H2O)(n)(2+), n = 30 and 32, suggesting that neither the water binding energy nor the recombination energy changes significantly for clusters of this size. The recombination energy is roughly estimated from theory to be about 4.5 eV. These results show that these ions are not significantly activated by inelastic non-capture collisions with electrons and that the vast majority of the recombination energy resulting from electron capture is converted into internal energy of the reduced precursor ions, indicating that these ions dissociate statistically.