Relative excitation functions for beam-foil populations in N II, N III, and N IV

The authors have measured relative populations of terms in N II, N III, and N IV, excited after the passage of ${\mathrm{N}}^{+}$ ions through a carbon foil, as a function of the ion energy [$0.3<E\left(\mathrm{MeV}\right)<\mathrm{}1.9\mathrm{}$]. These populations are obtained from the measurements of intensity of transitions emitted from these terms [$300<\mathrm{}\lambda \left(\AA \right)<\mathrm{}1800\mathrm{}$]. The variation of the ratio $R\left(E\right)\mathrm{}$ of the relative population $N_i$ ($i$ is the term excited in the ion of charge $q$) to the equilibrium charge-state fraction $P\left(q\right)\mathrm{}$ is studied as a function of the emergent beam energy. The grouping of the terms according to the number of electrons excited relative to the ground state shows that the $R\left(E\right)\mathrm{}$ functions are generally different for the different groups of terms. Within a given group of terms (singly or doubly excited terms), the slopes of the $R\left(E\right)\mathrm{}$ functions increase with the excitation energy of the term. These results are useful for classifying new lines observed in beam-foil spectroscopy.