Observation of ``magic numbers'' in the population distributions of the (NH3)n - 1NH + 2 and (NH3)nH + 2 cluster ions: Implications for cluster ion structures

We have measured the relative yields of the cluster ions (NH₃)_n-1NH⁺₂ and (NH₃)_nH⁺₂ 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 (NH₃)_n-1NH⁺₂ cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH⁺₂ daughter ion and an adjacent NH₃ solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n=5 and n=8 in the (NH₃)_nH⁺₂ cluster ion distribution is consistent with the production of the N₂H⁺₈ Rydberg radical cation via an intracluster ion-molecule reaction. Our experimental results are consistent with recent theoretical predictions concerning the structure and stability of N₂H⁺₈ ions.