The isotope shifts of long series of spectral lines in the optical spectra of the alkaline-earth elements Ca and Sr have been measured by Doppler-free, two-photon laser spectroscopy with a thermionic diode. The variation of transition isotope shifts with transition energy yields the level isotope shifts with reference to the ionization limit and how these shifts vary with the binding energy of the optically active electron. The largest isotope shifts of levels in these elements appear to arise from the specific-mass effect in the presence of state mixing.

• Received 23 July 1982

DOI:https://doi.org/10.1103/PhysRevA.28.2051