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: K³⁹ 4s²S1/2¹-∞=35009.83 cm^-1, Rb⁸⁵ 5s²S1/2²-∞=33691.02 cm^-1, Cs 6s²S1/2³-∞=31406.71 cm^-1. The hyperfine structure of the ground level 6s²S1/2 has been resolved in each member of the principal series of cesium.