Avoided curve crossing for the dissociation of the Rydberg NH4 radical into (NH3+H)

Abstract

Potential energy curves of the ground and low lying excited states for the dissociation of the Rydberg NH4 radical into (NH3+H) have been calculated using ab initio Hartree–Fock and singly excited configuration interaction methods with a large basis set including Rydberg basis functions. In the ground correlation curve, the ground (NH4+)(e−)3s radical diabatically correlates to the [H3N(3A1; n→3s)+H(2S)] and [NH3+(2A2″)+H−(1S)] asymptotes. An avoided curve crossing between two attractive diabatic states emerging from [H3N(3A1)+H(2S)] and [NH3+(2A2″)+H−(1S)] and a repulsive diabatic state emerging from an antibonding interaction of [NH3(1A1)+H(2S)] is found near the equilibrium geometry of NH4. The potential energy barrier of 0.59 eV on the ground correlation curve is found at R(NH)≃1.4 Å. The potential well is shallowly bound. In the excited curves, the curve crossings between the dissociative diabatic excited states of [(NH4+)(e−)Rydberg] and the repulsive diabatic states from the antibonding interactions of [NH3(1A1)+H(2S)] and [NH3(3A1; n→3s)+H(2S)] are found around R(NH)≃2.0 and 6.0 Å. The potential energy curves of the first and second excited A12 states are shallowly bound, while that of the third excited state is widely bound. A maximum position of the potential energy barrier of the ground correlation curve is located out of line of those of the excited states.