Fragmentation of NH3 dimers by electron impact ionization

Abstract

The fragmentation of ammonia dimers by electron impact ionization is investigated by preselecting the neutral species using scattering and spectroscopic methods. In the first experiment, the measured angular and velocity dependence of the ammonia cluster scattered from a helium beam is used to separate the dimer from the other contributions. In the second experiment, the rotationally resolved spectrum obtained by microwave electric resonance technique is used to label the dimer. The ratio of the ion intensities m=18 (NH⁺₄) to m=17 (NH⁺₃) amu is 1.3 and 1.8, respectively. In the case of scattering, the result is independent of the initial state distribution of the dimer. The remaining difference is attributed to the different ion sources. This remarkable result is explained in terms of simple models based on the structural change from the neutral to the ionized configuration and the fragmentation pattern of the monomer.