Forbidden Decays of Hydrogenlike and Heliumlike Argon

Four forbidden decay modes of hydrogenlike argon (Ar⁺¹⁷) and heliumlike argon (Ar⁺16) have been observed and the lifetimes of the states decaying via these modes have been measured by a beam-foil time-of-flight technique. The decays are the two-photon decay 2²S1/2→1²S1/2 of Ar⁺¹⁷, the two-photon decay 2¹S₀→1¹S₀ of Ar⁺¹⁶, the relativistically induced magnetic dipole decay 2³S₁→1¹S₀ of Ar⁺¹⁶, and the magnetic quadrupole decay 2³P₂→1¹S₀ of Ar⁺¹⁶. The measured lifetimes of the levels are τ(2²S1/2)=3.54±0.25 nsec, τ(2¹S₀)=2.3±0.3 nsec, τ(2³P₂=1.7±0.3 nsec, and τ(2³S₁)=172±30 nsec. From the agreement between the measured and calculated lifetimes it is possible to place an upper limit on the amount of parity impurity in the 2²S1/2-state wave function of hydrogenlike Ar⁺¹⁷. Writing ψ(2²S1/2)=Φ(2²S1/2)+εΦ(2²P1/2), where the Φ's are the Schr\"odinger solutions to the hydrogen atom, we obtain |ε|≤6×10^-4.