For plasma source ion implantation in a nitrogen discharge, there is a mixture of atomic and molecular species N+ and N2+, respectively. A time dependent fluid model has been developed, which models a plasma consisting of two species of cold collisionless ion fluids, assuming that the electrons are in thermal equilibrium. Following the application of a large, negative voltage pulse to the metal target, the density and velocity of the two ion species, and the sheath evolution for a range of possible ion mixtures are calculated.