Light-shift deviations observed in collinear laser-rf double-resonance spectroscopy

The energy difference between two hyperfine levels of the ground state, measured with high precision by collinear laser-rf double-resonance spectroscopy, is influenced by the light shift, depending on the intensity and frequency of the laser field. We report on a measurement of the change in rf transition frequency between the F=2 and F=3 levels of the 5d ${\mathrm{}}^2$ $D_{3/2}$ hyperfine multiplet of ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}_{}^{+}{}_{}{}^{}$ as a function of laser frequency. The observed results are in good agreement with theoretical calculations.