Specific mass shift of potassium 5P1/2 state

We have measured the isotope shift between 41K and 39K in the 4s1/2 → 5p1/2 transition at 405 nm using saturation spectroscopy. Our measured isotope shift is 456.1 ± 0.8 MHz, implying a residual isotope shift (sum of specific mass shift and field shift) of −52.7 ± 0.8 MHz. We deduce a specific mass shift of −40 ± 5 MHz, which would imply that the 5p1/2 state has a considerably larger specific mass shift than the 4p1/2 state. We have in addition measured the 5p1/2 hyperfine splitting for 41K.