Hyperfine Structure in Singly Ionized Praseodymium

Abstract

Wave-lengths and frequency separations of the fine-structure components of 173 spectral lines in singly ionized praseodymium are given. It appears, as may be seen from a reproduction of some of the fine-structure, Fig. 1, that each group of six fine-structure lines has the same general appearance but varies in total intensity and width. In some lines given below the intensities and frequency separations within each group decrease toward the red, while all of the rest show similar degradation toward the violet.Theoretical interpretation. These fine structures may be accounted for by assigning an angular momentum, $i=(\frac{5}{2})(\frac{h}{2\ensuremath{\pi}})$ to the nucleus of the praseodymium atom. This angular momentum space quantized with the total angular momentum of the outside electrons, $J$, yields for each energy level six components (provided $J\ensuremath{\geqq}\frac{5}{2}$), the actual separations between which are given by the strength of coupling between $i$ and $J$. Since a $6p$ electron in the atom of praseodymium is part of the time very close to the nucleus and part of the time outside of all of the other electrons, the strength of coupling for an electron configuration involving a single $6s$ electron should be very much greater than that for a $6s$ electron and still greater than that for a $5d$ or a $4f$ electron. According to the Bohr-Stoner scheme of the building up of the elements one should expect, and the fine-structure intervals confirm this, that practically all of the energy levels in Pr II have very large $J$ values.