A guided-ion beam study of the O+(4S) + NH3 system at hyperthermal energies

We have measured absolute cross section for the reaction of ground-state O(+) with ammonia at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) method. Measurements were also performed using ammonia-d3 to aid in mass assignments. The reaction is dominated at low collision energies by charge transfer; however, the cross section for this exothermic channel is rather small, decreasing sharply with energy from approximately 40 A(2) for normal ammonia at near-thermal energies and leveling off at 3.7 A(2) above 6 eV; the cross section is slightly smaller for ammonia-d3. Other channels, corresponding to the production of NH2(+) and NO(+), and possibly OH(+), were detected. The NO(+) channel, although nominally exothermic, is very small and exhibits a threshold at approximately 7 eV. Product recoil velocity distributions were also determined at selected collision energies, using GIB time-of-flight methods.