Isotope shift and hyperfine splitting of the $4s \rightarrow 5p$   transition in potassium
  We have investigated $4s\;^2S_{1/2} \rightarrow 5p\;^2P_{1/2}$ transition ($D_1$ line) of the potassium isotopes $^{39}$K, $^{40}$K, and $^{41}$K using Doppler-free laser saturation spectroscopy. Our measurements reveal the hyperfine splitting of the $5p\;^2P_{1/2}$ state of $^{40}$K and we have determined the specific mass shift and the nuclear field shift constants for the blue $D_1$ line. 
Alexandra Behrle, Marco Koschorreck, and Michael K\"ohl
physics.atom-ph cond-mat.quant-gas