Stability of the ammonium and methylammonium radicals from neutralized ion-beam spectroscopy

The stability of the ammonium radical (NH₄) was determined from measurements of the kinetic energy released in its fragmentation products following formation in a fast electron capture process: NH₄ ⁺ + Na → NH₄* + Na⁺. Scattering profiles for heavy (NH₃) and light (H) dissociation products were obtained from beam measurements with 5-16 keV NH₄⁺ ions. The existence of a predissociative barrier in the radical is inferred from edge structure and scattering continua in H atom profiles. The radical is bound with respect to a potential minimum but all of the isotopic species NH₄, NH₃D, NH₂D₂, and NHD₃ undergo rapid loss of H atoms and have ground states lying above their dissociation limits. The radical ND₄ has unusual stability with its ground state lying close to or below its dissociation limit. Dissociative lifetimes for stable and unstable states of ND₄ differ by at least two orders of magnitude. The possible significance of these observations on the interpretation of optical transitions involving the ground states of NH₄ and ND₄ are discussed. From these measurements the ionization potential of 4.73±0.06 eV for NH₄ is calculated. The stabilities of CH ₃NH₃ and CH₃ND₃ radicals and their dissociative pathways have also been investigated.