Transition in Potassium*

We propose measuring the isotope shift of the 4S to 5P_1/2 transition at 405 nm in potassium isotopes ³⁸K, ³⁹K, ⁴⁰K, and ⁴¹K using Doppler-free laser saturation spectroscopy. The narrower linewidth would improve statistical accuracy by a factor of 5. This would allow us to measure the hyperfine splitting and to deduce changes in the nuclear mean-square charge radii. Better frequency metrology is needed and would provide benchmarks for the modern atomic calculations [1]. We aim to improve upon the metrology of previous measurements [2, 3]. This method involves one beam for locking and another for sweeping over the resonance of laser-cooled atoms, thus a key challenge is controlling the laser frequency from drifting. A simple scan produces surprisingly good linearity between the piezo voltage and the frequency of the laser, accurate to within 2 MHz of 5 MHz scan. However, this is inadequate, as we aim for a tenfold improvement over previous measurements.

References:

[1] V. Katyal, A. Chakraborty, B. K. Sahoo, B. Ohayon, C. Y. Seng, M. Gorchtein, and J. Behr, “Testing for isospin symmetry breaking by combining isotope shift measurements with precise calculations in potassium,” Phys. Rev. A 111, 042813 (2025).

[2] L. J. S. Halloran, S. Fostner, E. Paradis, and J. A. Behr, “Specific mass shift of potassium 5P₁/₂ state,” Optics Communications 282, 554 (2009).

[3] A. Behrle, M. Koschorreck, and M. Köhl, “Isotope shift and hyperfine splitting of the 4s → 5p transition in potassium,” Phys. Rev. A 83, 052507 (2011).