Experimental determination of the relativistic fine-structure splitting in pionic Ti and Fe atoms

Using a high-resolution crystal spectrometer we have measured the relativistic angular-momentum splittings of the $5g-4f$ and $5f-4d$ transitions in pionic Ti and Fe atoms. The observed fine-structure splittings of 85.3±3.0 eV in ${\pi }^{-}$ Ti and 158.5±7.8 eV in ${\pi }^{-}$ Fe agree with the calculated splittings of 88.5 and 167.6 eV, respectively, arising from the Klein-Gordon equation and from small corrections due to vacuum polarization, strong interaction, and electron screening.