Exploring Relativistic Many-Body Recoil Effects in Highly Charged Ions

The relativistic recoil effect has been the object of experimental investigations using highly charged ions at the Heidelberg electron beam ion trap. Its scaling with the nuclear charge $Z$ boosts its contribution to a measurable level in the magnetic-dipole ($M1$) transitions of B- and Be-like Ar ions. The isotope shifts of $^{36}\mathrm{A}\mathrm{r}$ versus $^{40}\mathrm{A}\mathrm{r}$ have been detected with sub-ppm accuracy, and the recoil effect contribution was extracted from the $1{s}^{2}2{s}^{2}2p$ $^{2}P_{1/2}\ensuremath{-}^{2}P_{3/2}$ transition in ${\mathrm{Ar}}^{13+}$ and the $1{s}^{2}2s2p$ $^{3}P_{1}\ensuremath{-}^{3}P_{2}$ transition in ${\mathrm{Ar}}^{14+}$. The experimental isotope shifts of 0.00123(6) nm (${\mathrm{Ar}}^{13+}$) and 0.00120(10) nm (${\mathrm{Ar}}^{14+}$) are in agreement with our present predictions of 0.00123(5) nm (${\mathrm{Ar}}^{13+}$) and 0.00122(5) nm (${\mathrm{Ar}}^{14+}$) based on the total relativistic recoil operator, confirming that a thorough understanding of correlated relativistic electron dynamics is necessary even in a region of intermediate nuclear charges.