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: K^39 4s^2S₁₂^1-=35009. 83 cm^-1, Rb^85 5s^2S₁₂^2-=33691. 02 cm^-1, Cs 6s^2S₁₂^3-=31406. 71 cm^-1. The hyperfine structure of the ground level 6s^2S₁₂ has been resolved in each member of the principal series of cesium.