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We measured the isotope shift in the $^2$S$_{1/2}$-$^2$P$_{3/2}$ (D2)transition in singly-ionized calcium ions using photon recoil spectroscopy. Thehigh accuracy of the technique enables us to resolve the difference between theisotope shifts of this transition to the previously measured isotopic shifts ofthe $^2$S$_{1/2}$-$^2$P$_{1/2}$ (D1) line. This so-called splitting isotopeshift is extracted and exhibits a clear signature of field shift contributions.From the data we were able to extract the small difference of the field shiftcoefficient and mass shifts between the two transitions with high accuracy.This J-dependence is of relativistic origin and can be used to benchmark atomicstructure calculations. As a first step, we use several ab initio atomicstructure calculation methods to provide more accurate values for the fieldshift constants and their ratio. Remarkably, the high-accuracy value for theratio of the field shift constants extracted from the experimental data islarger than all available theoretical predictions.

Unexpectedly large difference of the electron density at the nucleus in the 4p $^2$P$_{1/2,3/2}$ fine-structure doublet of Ca$^+$