## Abstract

Nine Rydberg series have been observed in the far ultraviolet absorption spectrum of P$_{2}$. Four of these converge to the...(5$\sigma _{\text{g}}$)$^{2}$ (2$\pi _{\text{u}}$)$^{3}$, $^{2}\Pi _{\text{u}}$ (inv.) state of the ion which is established as being the ground state; four to the low-lying...(5$\sigma _{\text{g}}$) (2$\pi _{\text{u}}$)$^{4}$, A $^{2}\Sigma _{\text{g}}^{+}$ state and one to a newly identified (5$\sigma _{\text{g}}$) (2$\pi _{\text{u}}$)$^{3}$ $2\pi _{\text{g}}$, F$^{2}\Sigma _{\text{u}}^{+}$ state. The first ionization potential is found to be 85 229 $\pm $ 15 cm$^{-1}$ (10.567 $\pm $ 0.002 eV), which is the limit corresponding to the upper component $(2\Pi _{\frac{1}{2}})$ of the inverted X$^{2}\Pi _{\text{u}}$ state. The other limits are observed at 87 179 $\pm $ 2 cm$^{-1}$ (A $^{2}\Sigma _{\text{g}}^{+}$) and 125 225 $\pm $ 10 cm$^{-1}$ (F $^{2}\Sigma _{\text{u}}^{+}$). The series have been interpreted in terms of molecular orbitals and are found to involve excitation of $n\text{s}\sigma _{\text{g}},n\text{d}\sigma _{\text{g}},n\text{d}\pi _{\text{g}}\text{and}n\text{d}\delta _{\text{g}}$ for the X $^{2}\Pi _{\text{u}}$ core; $n\text{p}\sigma _{\text{u}},n\text{p}\pi _{\text{u}},n\text{f}\sigma _{\text{u}}\text{and}n\text{f}\pi _{\text{u}}$ for the A $^{2}\Sigma _{\text{g}}^{+}$ core and $n\text{s}\sigma _{\text{g}}$ for the F $^{2}\Sigma _{\text{u}}^{+}$ core. The evaluation and identification of the series limits enables the relative positions of the states of P$_{2}^{+}$ to be established. The dissociation energy of P$_{2}^{+}$ is estimated to be $4.98\pm 0.01$ eV.