Isotope shift measurements in the atomic spectrum of lanthanum (LaI)

Using the reactor-produced lanthanum isotope ₅₇ ¹³⁷ La, the highly enriched rare ₅₇ ¹³⁸ La and the stable ₅₇ ¹³⁹ La, the isotope shift has been measured in five lines of the La I-spectrum with the aid of a pressure-scanned Fabry-Pérot interferometer. The isotope shift data obtained show surprisingly large specific mass effects, arising from configuration mixing of levels with a 4f electron involved. The changes in mean square charge radiusδ〈r ²〉 of these nuclei extracted from the experimental isotope shift constants C_exp are compared with the corresponding values for the isotonic barium nuclei, where similar anomalies in the isotope shifts occur.