Theoretical and experimental analysis of ammonia ionic clusters produced by 252Cf fragment impact on an NH3 ice target

Positive and negatively charged ammonia clusters produced by the impact of Cf-252 fission fragments (FF) on an NH3 ice target have been examined theoretical and experimentally. The ammonia clusters generated by 252Cf FF show an exponential dependence of the cluster population on its mass, and the desorption yields for the positive (NH3)(n)NH4+ clusters are 1 order of magnitude higher than those for the negative (NH3)(n)NH2- clusters. The experimental population analysis of (NH3)(n)NH4+ ( n) 0- 18) and (NH3)(n)NH2- (n = 0-8) cluster series show a special stability at n = 4 and 16 and n = 2, 4, and 6, respectively. DFT/B3LYP calculations of the (NH3)(0-8)NH4+ clusters show that the structures of the more stable conformers follow a clear pattern: each additional NH3 group makes a new hydrogen bond with one of the hydrogen atoms of an NH3 unit already bound to the NH4+ core. For the (NH3)(0-8)NH2- clusters, the DFT/B3LYP calculations show that, within the calculation error, the more stable conformers follow a clear pattern for n = 1-6: each additional NH3 group makes a new hydrogen bond to the NH2- core. For n) 7 and 8, the additional NH3 groups bind to other NH3 groups, probably because of the saturation of the NH2- core. Similar results were obtained at the MP2 level of calculation. A stability analysis was performed using the commonly defined stability function En-1 + En+1 - 2E(n), where E is the total energy of the cluster, including the zero point correction energy (E = E-t + ZPE). The trend on the relative stability of the clusters presents an excellent agreement with the distribution of experimental cluster abundances. Moreover, the stability analysis predicts that the (NH3)(4)-NH4+ + and the even negative clusters [(NH3)(n)NH2-, n = 2, 4, and 6] should be the most stable ones, in perfect agreement with the experimental results.