Observation of "magic numbers" in the population distributions of the (NH₃)_n-1 NH₂⁺ and (NH ₃)_n H₂⁺ cluster ions: Implications for cluster ion structures

We have measured the relative yields of the cluster ions (NH ₃)_n-1NH₂⁺ and (NH₃) _nH₂⁺ produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy. Our observation of a magic number at n = 7 in the distribution of (NH ₃)_n-1NH₂⁺ cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH₂⁺ daughter ion and an adjacent NH ₃ solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n = 5 and n = 8 in the (NH ₃)_nH₂⁺ cluster ion distribution is consistent with the production of the N₂H₈⁺ Rydberg radical cation via an intracluster ion-molecule reaction. Our experimental results are consistent with recent theoretical predictions concerning the structure and stability of N₂H₈⁺ ions.