Measurement of the $1s2s{}_{}{}^1{}_{}{}^{}S-1s2p{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ Interval in Heliumlike Nitrogen

For a large range of $Z$ the $1s2s{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-1s2p{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ interval in heliumlike ions lies in a wavelength region accessible by infrared lasers. Measurements of this interval will enable exceptionally sensitive tests of calculations of relativistic corrections and QED effects (Lamb shifts) in two-electron ions. This Letter describes a precision measurement of this interval in ${\mathrm{N}}^{5+}$ using a fast ion beam and a C${\mathrm{O}}_2$ laser. Our result is compared with recent relativistic and QED calculations.