Measurements of Relativistic and Quantum Electrodynamic Effects in Heliumlike Argon Using Position-Sensitive Ultraviolet Spectroscopy.

The 1s2s ^3S_1 - 1s2p ^3P_0 and 1s2s ^3S_1 - 1s2p ^3P_2 fine-structure transition wave-lengths in helium-like argon have been measured with a precision of <30 ppm. The fluorescence from a foil-excited beam of argon ions was studied using a one-meter normal incidence vacuum -ultraviolet spectrometer equipped with a microchannel-plate position-sensitive photon detector. The wavelength results are 661.533(16) A and 559.944(16) A for the 2 ^3S_1 - 2 ^3P_0 and 2 ^3S_1 - 2 ^3P _2 transitions, respectively. The experimental values are sensitive to the recently calculated (Z alpha)^4-dependent relativistic corrections to the n = 2 state energies that represent a new benchmark in the theoretical understanding of the atomic structure of heliumlike ions. These measurements provide the highest-Z values of sufficient precision to test the uncalculated higher-order QED corrections to the heliumlike n = 2 state structure. The small discrepancy between these measurements and the accurate theoretical values suggests that the inclusion of uncalculated higher-order QED terms will reduce the theoretical 1s2s ^3S_1 state energy for Ar ^{16{+}} by 4.5(2.3) times 10^{{-}5} a.u.