Stability of the Ground and Low-Lying Vibrational States of the Ammonium Radical

The tetrahedral ammonium radicals NH4 and ND4 are exotic neutral molecules that can be produced by charge exchange (CE) neutralization of the ammonium cation. In the present study, the stability of these radicals was measured by translational spectroscopy on the NH3/ND3 + H/D fragments resulting from CE between NH4+/ND4+ and cesium. The vibrationally resolved spectrum reveals the energy of the 3s((2)A(1)) ground state of the ammonium radical relative to ground-state NH3 + H dissociation products as well as the energies for the singly excited nu(1) and nu(2) modes of the NH4 isotopologues. Using these values, a lower limit for the heat of formation of NH4, Delta H-f,H-0k degrees = (189.7 +/- 0.4) kJ/mol has been determined, as well as an adiabatic ionization energy of (4.62 +/- 0.01) eV for NH4, in good agreement with recent predictions.