Anomalies in the optical isotope shifts of samarium

Optical isotope shifts have been measured in eight spectral lines of samarium. The technique, which used tunable dye lasers and optical heterodyne methods, yielded uncertainties of +or-200 kHz. King plots involving two of the spectral lines with any of the other six show significant deviations from straight lines. The two anomalous transitions have the same upper level and show deviations of the isotope pair 154Sm-152Sm of 32 MHz from the expected values, other pairs deviating by only about 1 MHz. Effects due to fourth and higher moments of the nuclear-charge distribution do not appear able to explain these deviations within the framework of current theory.