Measurements of the $1s2s{}^1{S}_0$–$1s2p{}^3{P}_{1,0}$ transitions in heliumlike nitrogen

Using a Doppler-tuned fast ion-beam–laser technique, with co- and counterpropagating laser beams, the intercombination $1s2s{}^1{S}_0$–$1s2p{}^3{P}_{1,F}$ transitions in ${}^{14}{\mathrm{N}}^{5+}$ and ${}^{15}{\mathrm{N}}^{5+}$ have been measured to 0.7 ppm. This precision is equivalent to 20 ppm of the two-electron Lamb shift and is more than two orders of magnitude more precise than current relativistic and QED theoretical results at this $Z$. A comparison is made with theory for the hyperfine structure and isotope shift. Additionally, using a single copropagating laser beam, the differences between the $2{}^1{S}_0$–$2{}^3{P}_{1,F=1}$ and $2{}^1{S}_0$–$2{}^3{P}_0$ intervals in ${}^{14}{\mathrm{N}}^{5+}$ and between the $2{}^1{S}_0$–$2{}^3{P}_{1,F=3/2}$ and $2{}^1{S}_0$–$2{}^3{P}_0$ intervals in ${}^{15}{\mathrm{N}}^{5+}$ have also been measured. From the former we obtain an improved result for the ${}^{14}{\mathrm{N}}^{5+}$ $2{}^3{P}_1$–$2{}^3{P}_0$ fine structure.