Isotope shifts of energy levels in40K and39K

Abstract
The isotope shifts of several energy levels in the mildly radioactive isotope 40 of potassium with respect to the stable isotopes 39K and 41K have been measured by Doppler-free two-photon spectroscopy on transitions to highly excited Rydberg states of atomic potassium. The contribution of mass-dependent effects ('specific mass shift') to the isotope shifts of the measured levels is assessed. Volume effects are found in the isotope shift of the ground state.