Precision measurement of relativistic and QED effects in heliumlike beryllium

We have used fast-ion-beam laser spectroscopy to measure the 1s2${\mathit{s}}^3$s-1s2${\mathit{p}}^3$ ${\mathit{P}}^{\mathit{o}}$ intervals in Be iii to an absolute accuracy of 1 part in ${10}^8$. Our results are (in ${\mathrm{cm}}^{\mathrm{-}1}$): σ(1s2${\mathit{s}}^3$ ${\mathit{S}}_1$-1s2${\mathit{p}}^3$ ${\mathit{P}}_0^{\mathit{o}}$=28 864.6120(4), σ(1s2${\mathit{s}}^3$ ${\mathit{S}}_{1\mathrm{-}}$1s2${\mathit{p}}^3$ ${\mathit{P}}_1^{\mathit{o}}$=26 853.0534(3), σ(1s2${\mathit{s}}^3$ ${\mathit{S}}_{1\mathrm{-}}$1s2${\mathit{p}}^3$ ${\mathit{P}}_2^{\mathit{o}}$) =26 867.9484(3). These results are an improvement over previous measurements by nearly 3 orders of magnitude and show evidence of uncalculated O(${\mathrm{\alpha }}^4$ ${\mathit{Z}}^4$) relativistic corrections. Once those terms are computed, our measurement will allow QED calculations in two-electron atoms to be tested at the 100 ppm level.