AMMONIUM-NITRATE CLUSTER IONS

Sputtering of condensed-phase ammonium nitrate yields many positive an d negative cluster ion series derived from different ionic cores. The cluster cores are surrounded by varying numbers of ammonium nitrate mo nomer units. Most interesting is the extensive series of negative clus ter ions of the form [(NH4NO3)(n)NO3](-), n greater than or equal to 3 . The corresponding positive clusters, [(NH4NO3)(n)NH4](+), are also v ery extensive but also include the smallest ions, n = 1 and 2. Collisi on-induced dissociation of mass-selected cluster ions suggests that th e first two members of the negative series, n = 1 and n = 2, are not d etected because they rearrange and lose one or more ammonia molecules. Gradient-density-functional calculations using two different function als predict that NH4NO3 is strongly hydrogen bonded and that [(NH4NO3) NO3](-) has no hydrogen bonds. This is consistent with this ion rearra nging by loss of NH3 to form the strongly hydrogen-bonded ion [H(NO3)( 2)](-). Rearrangements involving loss of ammonia molecules in the nega tive-ion spectrum and nitric acid molecules in the positive-ion spectr um lead to a rich variety of other, less extensive, series of sputtere d ions from this complex solid. Both relative gradient-density-functio nal energies correlate well with whether or not various ions are obser vable experimentally.