FINE-STRUCTURE ENERGIES FOR THE 1S2S 3S-1S2P 3P TRANSITION IN HELIUMLIKE AR16+

We have measured the 1[ital s]2[ital s] [sup 3][ital S][sub 1]--1[ital s]2[ital p] [sup 3][ital P][sub 0] and 1[ital s]2[ital s] [sup 3][ital S][sub 1]--1[ital s]2[ital p] [sup 3][ital P][sub 2] fine-structure transition wavelengths in the heliumlike Ar[sup 16+] ion using the beam-foil spectroscopy technique with position-sensitive photon detection. The wavelength results are 559.944(16) A for the [ital J]=1--2 transition and 661.533(18) A for the [ital J]=1--0 transition, which correspond to measurement precisions of about 2[times]10[sup [minus]5] a.u. in the transition energies. These values are sensitive to the relativistic contributions in the transition energies to [lt]500 ppm and to the total QED corrections to [lt]0.5%. The measurements confirm the significance of ([ital Z][alpha])[sup 4]-dependent relativistic corrections to the [ital n]=2 state fine structures suggested by recent calculations. The small remaining discrepancies between these measurements and the accurate theoretical values suggest that the magnitude of uncalculated higher-order QED terms is about 0.15([ital Z][alpha])[sup 4] a.u. Our experimental results disagree with an earlier recoil-ion measurement for the [sup 3][ital S][sub 1]-- [sup 3][ital P][sub 0] transition wavelength.