The Principal Series of Potassium, Rubidium, and Cesium in Absorption

The principal series of potassium, rubidium, and cesium in absorption have been measured. The doublet fine-structure has been resolved out to $17p$ in potassium, to $26p$ in rubidium, and to $21p$ in cesium; the doublet fine-structure interval varies in each spectrum inversely as the cube of the effective principal quantum number. In cesium, the doublet intensity ratio, instead of having a maximum, increases with increasing $n$ as far as it can be followed. Higher series members, with their doublet fine-structure unresolved, have been measured to $79p$ in potassium, to $77p$ in rubidium, and to $73p$ in cesium. The series limits, calculated from the lowest hyperfine structure levels, are: ${\mathrm{K}}^{39}$ $4s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^1-\infty =35009.83\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\mathrm{Rb}}^{85}$ $5s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^2-\infty =33691.02\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, Cs $6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^3-\infty =31406.71\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The hyperfine structure of the ground level $6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ has been resolved in each member of the principal series of cesium.