Experimental Optical Isotope Shifts and Nuclear Charge Radii of Potassium Isotopes

The isotope shifts of atomic transitions in potassium provide access to changes in nuclear charge radii and are also essential for efficient trapping and polarization of short-lived isotopes used in precision β-decay studies. For ³⁷K, which being a mirror nucleus, is of particular interest for tests of weak interactions, the optical isotope shift in the 4S → 4P_1/2 transition has only been measured to an accuracy of a few MegaHertz in previous work, on the order of the natural line width of ³⁹K, limiting both nuclear-structure extraction and optimal laser-cooling performance.



We report ongoing work to improve measurements of the optical isotope shifts of potassium isotopes. To determine the optical isotope shifts between the isotopes ³⁷K and ⁴¹K, we lock our optical pumping laser to a stable ³⁹K 4S → 4P_1/2 cross-over transition using saturated absorption spectroscopy (SAS) and apply frequency offsets corresponding to the relative isotope shifts of alternating laser-trapped ³⁷K and ⁴¹K atoms. We also plan a more precise determination of the 4S → 5P_1/2 transition frequency in ³⁸K isomer.