Dissociative electron attachment resonances in ammonia: A velocity slice imaging based study

Abstract

Negative ion resonance states of ammonia are accessed upon capture of electrons with energy 5.5 eV and 10.5 eV, respectively. These resonance states dissociate to produce H⁻ and NH ₂⁻ fragment anions via different fragmentation channels. Using the velocity slice imaging technique, we measured the angular and kinetic energy distribution of the fragment H⁻ and NH ₂⁻ anions with full 0–2π angular coverage across the two resonances. The scattered H⁻ ions at both resonances show variation in their angular distribution as a function of the kinetic energy indicating geometric rearrangement of NH ₃^−* ion due to internal excitations and differ from the equilibrium geometry of the neutral molecule. The second resonance at 10.5 eV shows strong forward-backward asymmetry in the scattering of H⁻ and NH 2⁻ fragment ions. Based on the angular distributions of the H⁻ ions, the symmetry of the resonances at 5.5 eV and 10 .5 eV are determined to be A₁ and E, respectively, within C_3v geometry.