Photodetachment spectroscopy near the lowest threshold of the O$^{-}$ ion

Numerous experiments have investigated photodetachment spectroscopy in a magnetic field at the $^{2}$P$_{3/2 }\to \quad ^{3}$P$_{2}$ threshold of ions such as S$^{-}$ and O$^{-}$. The threshold energy is known as the atom's \textit{electron affinity}. In this work we have investigated detachment at the lowest-lying threshold for the O$^{-}$ ion, the $^{2}$P$_{1/2 }\to \quad ^{3}$P$_{2}$ threshold. Our experimental apparatus includes a Penning ion trap in which the ions are created, trapped and stored, and a single-mode, amplified diode laser. Our observations yield a quantitative measurement for the threshold energy and an indirect measurement for the spin-orbit splitting of the ion.