Imaging a multidimensional multichannel potential energy surface: Photodetachment of H^-(NH₃) and NH₄^-

Probes of the Born-Oppenheimer potential energy surfaces governing polyatomic molecules often rely on spectroscopy for the bound regions or collision experiments in the continuum. A combined spectroscopic and half-collision approach to image nuclear dynamics in a multidimensional and multichannel system is reported here. The Rydberg radical NH₄ and the double Rydberg anion NH₄^- represent a polyatomic system for benchmarking electronic structure and nine-dimensional quantum dynamics calculations. Photodetachment of the H^-(NH₃) ion-dipole complex and the NH₄^- DRA probes different regions on the neutral NH₄ PES. Photoelectron energy and angular distributions at photon energies of 1.17, 1.60, and 2.33 eV compare well with quantum dynamics. Photoelectron-photofragment coincidence experiments indicate dissociation of the nascent NH₄ Rydberg radical occurs to H + NH₃ with a peak kinetic energy of 0.13 eV, showing the ground state of NH₄ to be unstable, decaying by tunneling-induced dissociation on a time scale beyond the present scope of multidimensional quantum dynamics.