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

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((3)A(1); n --> 3s) + H(S-2)] and [NH3+((2)A(2) '') + H-(S-1)] asymptotes. An avoided curve crossing between two attractive diabatic states emerging from [H3N((3)A(1))+ H(S-2)] and [NH3+((2)A(2) '') + H-(S-1)] and a repulsive diabatic state emerging from an antibonding interaction of [NH3((1)A(1)) + 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) similar or equal to 1.4 Angstrom. 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((1)A(1)) + H(S-2)] and [NH3((3)A(1); n --> (3)s) + H(S-2)] are found around R-(NH) similar or equal to 2.0 and 6.0 Angstrom. The potential energy curves of the first and second excited (2)A(1) 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. (C) 1997 American Institute of Physics. [S0021-9606(97)01140-9].